drm/i915: wait for a vblank to pass after tv detect

Otherwise the hw will get confused and result in a black screen.

This regression has been most likely introduce in

commit 974b93315b2213b74a42a87e8a9d4fc8c0dbe90c
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Sun Sep 5 00:44:20 2010 +0100

    drm/i915/tv: Poll for DAC state change

That commit replace the first msleep(20) with a busy-loop, but failed
to keep the 2nd msleep around. Later on we've replaced all these
msleep(20) by proper vblanks.

For reference also see the commit in xf86-video-intel:

commit 1142be53eb8d2ee8a9b60ace5d49f0ba27332275
Author: Jesse Barnes <jbarnes@hobbes.lan>
Date:   Mon Jun 9 08:52:59 2008 -0700

    Fix TV programming:  add vblank wait after TV_CTL writes

    Fxies FDO bug #14000; we need to wait for vblank after
    writing TV_CTL or following "DPMS on" calls may not actually enable the output.

v2: As suggested by Chris Wilson, add a small comment to ensure that
no one accidentally removes this vblank wait again - there really
seems to be no sane explanation for why we need it, but it is
required.

Launchpad: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/763688
Reported-and-Tested-by: Robert Lowery <rglowery@exemail.com.au>
Cc: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Cc: stable@vger.kernel.org
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_tv.c

@@ -1194,6 +1194,11 @@ intel_tv_detect_type(struct intel_tv *intel_tv,
 
 	I915_WRITE(TV_DAC, save_tv_dac & ~TVDAC_STATE_CHG_EN);
 	I915_WRITE(TV_CTL, save_tv_ctl);
+	POSTING_READ(TV_CTL);
+
+	/* For unknown reasons the hw barfs if we don't do this vblank wait. */
+	intel_wait_for_vblank(intel_tv->base.base.dev,
+			      to_intel_crtc(intel_tv->base.base.crtc)->pipe);
 
 	/* Restore interrupt config */
 	if (connector->polled & DRM_CONNECTOR_POLL_HPD) {