drm/amd/display: VGA black screen from s3 when attached to hook

[Description] For MST, DC already notify MST sink for MST mode, DC stll
check DP SINK DPCD register to see if MST enabled. DP RX firmware may
not handle this properly.
Signed-off-by: NHersen Wu <hersenxs.wu@amd.com>
Reviewed-by: NTony Cheng <Tony.Cheng@amd.com>
Acked-by: NHarry Wentland <harry.wentland@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -1465,7 +1465,7 @@ void decide_link_settings(struct dc_stream_state *stream,
 	/* MST doesn't perform link training for now
 	 * TODO: add MST specific link training routine
 	 */
-	if (is_mst_supported(link)) {
+	if (stream->signal == SIGNAL_TYPE_DISPLAY_PORT_MST) {
 		*link_setting = link->verified_link_cap;
 		return;
 	}