drm/amd/display: Only update link settings after successful MST link train

[WHY]
Currently driver reduces verified link caps on DPIA devices if a link is
trained at a link rate below the max rate verified during link detection.
This blocks high bandwidth modes after setting a low bandwidth mode.

[HOW]
Only update link rate after a successful link train if link is MST.

Reviewed-by: Mustapha Ghaddar <[e-mail]>
Acked-by: Alan Liu <[e-mail]>
Signed-off-by: Michael Strauss <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 {
 	/* TODO: add periodic detection implementation */
 }
+
+void dm_helpers_dp_mst_update_branch_bandwidth(
+		struct dc_context *ctx,
+		struct dc_link *link)
+{
+	// TODO
+}
+

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

···
 		link->preferred_link_setting.link_rate = LINK_RATE_UNKNOWN;
 	}
 
+	if (link->connector_signal == SIGNAL_TYPE_DISPLAY_PORT &&
+			link->type == dc_connection_mst_branch)
+		dm_helpers_dp_mst_update_branch_bandwidth(dc->ctx, link);
+
 	/* Retrain now, or wait until next stream update to apply */
 	if (skip_immediate_retrain == false)
 		dc_link_set_preferred_link_settings(dc, &link->preferred_link_setting, link);

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

···
 					/* Update verified link settings to current one
 					 * Because DPIA LT might fallback to lower link setting.
 					 */
-					link->verified_link_cap.link_rate = link->cur_link_settings.link_rate;
-					link->verified_link_cap.lane_count = link->cur_link_settings.lane_count;
+					if (stream->signal == SIGNAL_TYPE_DISPLAY_PORT_MST) {
+						link->verified_link_cap.link_rate = link->cur_link_settings.link_rate;
+						link->verified_link_cap.lane_count = link->cur_link_settings.lane_count;
+						dm_helpers_dp_mst_update_branch_bandwidth(link->ctx, link);
+					}
 				}
 			} else {
 				status = dc_link_dp_perform_link_training(link,

drivers/gpu/drm/amd/display/dc/dm_helpers.h

···
 bool dm_helpers_dp_mst_stop_top_mgr(
 		struct dc_context *ctx,
 		struct dc_link *link);
+
+void dm_helpers_dp_mst_update_branch_bandwidth(
+		struct dc_context *ctx,
+		struct dc_link *link);
+
 /**
  * OS specific aux read callback.
  */