drm/i915: unbreak lastclose for failed driver init

We now refuse to load on gen6+ if kms is not enabled:

commit 26394d9251879231b85e6c8cf899fa43e75c68f1
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Mar 26 21:33:18 2012 +0200

    drm/i915: refuse to load on gen6+ without kms

Which results in the drm core calling our lastclose function to clean
up the mess, but that one is neatly broken for such failure cases
since kms has been introduced in

commit 79e539453b34e35f39299a899d263b0a1f1670bd
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Nov 7 14:24:08 2008 -0800

    DRM: i915: add mode setting support

Reported-and-tested-by: Paulo Zanoni <przanoni@gmail.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_dma.c

@@ -1781,7 +1781,13 @@ void i915_driver_lastclose(struct drm_device * dev)
 {
 	drm_i915_private_t *dev_priv = dev->dev_private;
 
-	if (!dev_priv || drm_core_check_feature(dev, DRIVER_MODESET)) {
+	/* On gen6+ we refuse to init without kms enabled, but then the drm core
+	 * goes right around and calls lastclose. Check for this and don't clean
+	 * up anything. */
+	if (!dev_priv)
+		return;
+
+	if (drm_core_check_feature(dev, DRIVER_MODESET)) {
 		intel_fb_restore_mode(dev);
 		vga_switcheroo_process_delayed_switch();
 		return;