drm/i915: Only apply DPMS to the encoder if enabled

The current test for an attached enabled encoder fails if we have
multiple connectors aliased to the same encoder - both connectors
believe they own the enabled encoder and so we attempt to both enable
and disable DPMS on the encoder, leading to hilarity and an OOPs:

[  354.803064] WARNING: CPU: 0 PID: 482 at
/usr/src/linux/dist/3.11.2/drivers/gpu/drm/i915/intel_display.c:3869 intel_modeset_check_state+0x764/0x770 [i915]()
[  354.803064] wrong connector dpms state
[  354.803084] Modules linked in: nfsd auth_rpcgss oid_registry exportfs nfs lockd sunrpc xt_nat iptable_nat nf_nat_ipv4 nf_nat xt_limit xt_LOG xt_tcpudp nf_conntrack_ipv4 nf_defrag_ipv4 ipt_REJECT ipv6 xt_recent xt_conntrack nf_conntrack iptable_filter ip_tables x_tables snd_hda_codec_realtek snd_hda_codec_hdmi x86_pkg_temp_thermal snd_hda_intel coretemp kvm_intel snd_hda_codec i915 kvm snd_hwdep snd_pcm_oss snd_mixer_oss crc32_pclmul snd_pcm crc32c_intel e1000e intel_agp igb ghash_clmulni_intel intel_gtt aesni_intel cfbfillrect aes_x86_64 cfbimgblt lrw cfbcopyarea drm_kms_helper ptp video thermal processor gf128mul snd_page_alloc drm snd_timer glue_helper 8250_pci snd pps_core ablk_helper agpgart cryptd sg soundcore fan i2c_algo_bit sr_mod thermal_sys 8250 i2c_i801 serial_core
hwmon cdrom i2c_core evdev button
[  354.803086] CPU: 0 PID: 482 Comm: kworker/0:1 Not tainted 3.11.2 #1
[  354.803087] Hardware name: Supermicro X10SAE/X10SAE, BIOS 1.00 05/03/2013 [  354.803091] Workqueue: events console_callback
[  354.803092]  0000000000000009 ffff88023611db48 ffffffff814048ac ffff88023611db90
[  354.803093]  ffff88023611db80 ffffffff8103d4e3 ffff880230d82800 ffff880230f9b800
[  354.803094]  ffff880230f99000 ffff880230f99448 ffff8802351c0e00 ffff88023611dbe0
[  354.803094] Call Trace:
[  354.803098]  [<ffffffff814048ac>] dump_stack+0x54/0x8d
[  354.803101]  [<ffffffff8103d4e3>] warn_slowpath_common+0x73/0x90
[  354.803103]  [<ffffffff8103d547>] warn_slowpath_fmt+0x47/0x50
[  354.803109]  [<ffffffffa089f1be>] ? intel_ddi_connector_get_hw_state+0x5e/0x110 [i915]
[  354.803114]  [<ffffffffa0896974>] intel_modeset_check_state+0x764/0x770 [i915]
[  354.803117]  [<ffffffffa08969bb>] intel_connector_dpms+0x3b/0x60 [i915]
[  354.803120]  [<ffffffffa037e1d0>] drm_fb_helper_dpms.isra.11+0x120/0x160 [drm_kms_helper]
[  354.803122]  [<ffffffffa037e24e>] drm_fb_helper_blank+0x3e/0x80 [drm_kms_helper]
[  354.803123]  [<ffffffff812116c2>] fb_blank+0x52/0xc0
[  354.803125]  [<ffffffff8121e04b>] fbcon_blank+0x21b/0x2d0
[  354.803127]  [<ffffffff81062243>] ? update_rq_clock.part.74+0x13/0x30
[  354.803129]  [<ffffffff81047486>] ? lock_timer_base.isra.30+0x26/0x50
[  354.803130]  [<ffffffff810472b2>] ? internal_add_timer+0x12/0x40
[  354.803131]  [<ffffffff81047f48>] ? mod_timer+0xf8/0x1c0
[  354.803133]  [<ffffffff81266d61>] do_unblank_screen+0xa1/0x1c0
[  354.803134]  [<ffffffff81268087>] poke_blanked_console+0xc7/0xd0
[  354.803136]  [<ffffffff812681cf>] console_callback+0x13f/0x160
[  354.803137]  [<ffffffff81053258>] process_one_work+0x148/0x3d0
[  354.803138]  [<ffffffff81053f19>] worker_thread+0x119/0x3a0
[  354.803140]  [<ffffffff81053e00>] ? manage_workers.isra.30+0x2a0/0x2a0
[  354.803141]  [<ffffffff8105994b>] kthread+0xbb/0xc0
[  354.803142]  [<ffffffff81059890>] ? kthread_create_on_node+0x120/0x120
[  354.803144]  [<ffffffff8140b32c>] ret_from_fork+0x7c/0xb0
[  354.803145]  [<ffffffff81059890>] ? kthread_create_on_node+0x120/0x120

This regression goes back to the big modeset rework and the conversion
to the new dpms helpers which started with:

commit 5ab432ef4997ce32c9406721b37ef6e97e57dae1
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sat Jun 30 08:59:56 2012 +0200

    drm/i915/hdmi: convert to encoder->disable/enable

Fixes: igt/kms_flip/dpms-off-confusion
Reported-and-tested-by: Wakko Warner <wakko@animx.eu.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68030
Link:  http://lkml.kernel.org/r/20130928185023.GA21672@animx.eu.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
[danvet: Add regression citation, mention the igt testcase this fixes
and slap a cc: stable on the patch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -3941,8 +3941,6 @@ static void intel_connector_check_state(struct intel_connector *connector)
  * consider. */
 void intel_connector_dpms(struct drm_connector *connector, int mode)
 {
-	struct intel_encoder *encoder = intel_attached_encoder(connector);
-
 	/* All the simple cases only support two dpms states. */
 	if (mode != DRM_MODE_DPMS_ON)
 		mode = DRM_MODE_DPMS_OFF;
@@ -3953,10 +3951,8 @@ void intel_connector_dpms(struct drm_connector *connector, int mode)
 	connector->dpms = mode;
 
 	/* Only need to change hw state when actually enabled */
-	if (encoder->base.crtc)
-		intel_encoder_dpms(encoder, mode);
-	else
-		WARN_ON(encoder->connectors_active != false);
+	if (connector->encoder)
+		intel_encoder_dpms(to_intel_encoder(connector->encoder), mode);
 
 	intel_modeset_check_state(connector->dev);
 }