drm/nouveau/dispnv50: fix runtime pm imbalance on error

pm_runtime_get_sync() increments the runtime PM usage counter even
the call returns an error code. Thus a pairing decrement is needed
on the error handling path to keep the counter balanced.
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/dispnv50/disp.c

@@ -1137,8 +1137,10 @@ nv50_mstc_detect(struct drm_connector *connector,
 		return connector_status_disconnected;
 
 	ret = pm_runtime_get_sync(connector->dev->dev);
-	if (ret < 0 && ret != -EACCES)
+	if (ret < 0 && ret != -EACCES) {
+		pm_runtime_put_autosuspend(connector->dev->dev);
 		return connector_status_disconnected;
+	}
 
 	ret = drm_dp_mst_detect_port(connector, ctx, mstc->port->mgr,
 				     mstc->port);