drm/radeon: Fix a missing check bug in radeon_dp_mst_detect()

In radeon_dp_mst_detect(), We should check whether or not @connector
has been unregistered from userspace. If the connector is unregistered,
we should return disconnected status.

Fixes: 9843ead ("drm/radeon: add DisplayPort MST support (v2)")
Signed-off-by: Yingjie Wang <wangyingjie55@126.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_dp_mst.c

@@ -241,6 +241,9 @@ radeon_dp_mst_detect(struct drm_connector *connector,
 		to_radeon_connector(connector);
 	struct radeon_connector *master = radeon_connector->mst_port;
 
+	if (drm_connector_is_unregistered(connector))
+		return connector_status_disconnected;
+
 	return drm_dp_mst_detect_port(connector, ctx, &master->mst_mgr,
 				      radeon_connector->port);
 }