drm/i915: fully switch off overlay when not in use

Now that the cache flushing of the memory based overlay regs works,
we can safely switch off the overlay. Beforehand it was only disabled
(like in userspace).
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_overlay.c

@@ -323,7 +323,6 @@ static int intel_overlay_off(struct intel_overlay *overlay)
 	}
 
 	/* turn overlay off */
-	/* this is not done in userspace!
 	BEGIN_LP_RING(6);
         OUT_RING(MI_FLUSH);
         OUT_RING(MI_NOOP);
@@ -338,7 +337,7 @@ static int intel_overlay_off(struct intel_overlay *overlay)
 		DRM_ERROR("intel overlay: ring sync failed, hw likely wedged\n");
 		overlay->hw_wedged = 1;
 		return ret;
-	}*/
+	}
 
 	overlay->active = 0;