drm/i915: hold ref on flip object until it completes

This will prevent things from falling over if the user frees the flip
buffer before we complete the flip, since we'll hold an internal
reference.
Reported-by: NKristian Høgsberg <krh@bitplanet.net>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -4094,6 +4094,7 @@ static void intel_unpin_work_fn(struct work_struct *__work)
 
 	mutex_lock(&work->dev->struct_mutex);
 	i915_gem_object_unpin(work->old_fb_obj);
+	drm_gem_object_unreference(work->pending_flip_obj);
 	drm_gem_object_unreference(work->old_fb_obj);
 	mutex_unlock(&work->dev->struct_mutex);
 	kfree(work);
@@ -4221,8 +4222,9 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 		return ret;
 	}
 
-	/* Reference the old fb object for the scheduled work. */
+	/* Reference the objects for the scheduled work. */
 	drm_gem_object_reference(work->old_fb_obj);
+	drm_gem_object_reference(obj);
 
 	crtc->fb = fb;
 	i915_gem_object_flush_write_domain(obj);