loginDialog: hide user list when starting timed login

If the timeout fires and we're starting timed login, we
should hide the user list, in case the login process
needs to show the user any messages on the auth prompt.

js/gdm/loginDialog.js

@@ -1083,6 +1083,7 @@ var LoginDialog = GObject.registerClass({
 
                      () => {
                          this._timedLoginBatch = null;
+                         this._hideUserList();
                          this._greeter.call_begin_auto_login_sync(userName, null);
                      }];