drm/amd/display: Add MST debug message when link detection fails

[WHY & HOW]
dc_link_detect returns a boolean value which can be used to print debug
messages when it fails.

This fixes 1 CHECKED_RETURN issue reported by Coverity.
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -3804,9 +3804,12 @@ static int trigger_hpd_mst_set(void *data, u64 val)
 			if (aconnector->dc_link->type == dc_connection_mst_branch &&
 			    aconnector->mst_mgr.aux) {
 				mutex_lock(&adev->dm.dc_lock);
-				dc_link_detect(aconnector->dc_link, DETECT_REASON_HPD);
+				ret = dc_link_detect(aconnector->dc_link, DETECT_REASON_HPD);
 				mutex_unlock(&adev->dm.dc_lock);
 
+				if (!ret)
+					DRM_ERROR("DM_MST: Failed to detect dc link!");
+
 				ret = drm_dp_mst_topology_mgr_set_mst(&aconnector->mst_mgr, true);
 				if (ret < 0)
 					DRM_ERROR("DM_MST: Failed to set the device into MST mode!");