drm: disconnect plane from fb/crtc when disabled

Since plane->fb and plane->crtc are set in drm_mode_setplane()
after update_plane(), They should be cleared after disable().

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index f259a25..7710bcb 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -348,6 +348,9 @@
 			ret = plane->funcs->disable_plane(plane);
 			if (ret)
 				DRM_ERROR("failed to disable plane with busy fb\n");
+			/* disconnect the plane from the fb and crtc: */
+			plane->fb = NULL;
+			plane->crtc = NULL;
 		}
 	}