drm/nouveau/kms: take mode_config mutex in connector hotplug path

commit 0a882cad upstream.

fdo#93634
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
[ kamal: backport to 3.13-stable: context ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/gpu/drm/nouveau/nouveau_connector.c

@@ -925,10 +925,12 @@ nouveau_connector_hotplug_work(struct work_struct *work)
 	NV_DEBUG(drm, "%splugged %s\n", plugged ? "" : "un",
 		 drm_get_connector_name(connector));
 
+	mutex_lock(&drm->dev->mode_config.mutex);
 	if (plugged)
 		drm_helper_connector_dpms(connector, DRM_MODE_DPMS_ON);
 	else
 		drm_helper_connector_dpms(connector, DRM_MODE_DPMS_OFF);
+	mutex_unlock(&drm->dev->mode_config.mutex);
 
 	drm_helper_hpd_irq_event(dev);
 }