drm/amd/display: Check aux channel before MST resume

It is to fix: MST display failed to resume from S3

At the beginning of resume from S3, need to check if mgr->aux is
NULL. Fake MST encoder doesn't have real aux channel.
Signed-off-by: NJerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: NRoman Li <Roman.Li@amd.com>
Acked-by: NHarry Wentland <harry.wentland@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -520,7 +520,8 @@ static int detect_mst_link_for_all_connectors(struct drm_device *dev)
 
 	list_for_each_entry(connector, &dev->mode_config.connector_list, head) {
 		aconnector = to_amdgpu_dm_connector(connector);
-		if (aconnector->dc_link->type == dc_connection_mst_branch) {
+		if (aconnector->dc_link->type == dc_connection_mst_branch &&
+		    aconnector->mst_mgr.aux) {
 			DRM_DEBUG_DRIVER("DM_MST: starting TM on aconnector: %p [id: %d]\n",
 					aconnector, aconnector->base.base.id);