drm: plane: Clear plane.crtc and plane.fb after disable_plane()

These are the only indication to user space that the plane was disabled.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc.c

@@ -1684,6 +1684,8 @@ int drm_mode_setplane(struct drm_device *dev, void *data,
 	/* No fb means shut it down */
 	if (!plane_req->fb_id) {
 		plane->funcs->disable_plane(plane);
+		plane->crtc = NULL;
+		plane->fb = NULL;
 		goto out;
 	}