xwayland: Change randr_output status when call xwl_output_remove()

The function xwl_output_remove() is called when removing a monitor, but
the actual status of the RandR output does not change.

So, when RRTellChanged() is called from update_screen_size(), it won't
have the output connection status up to date in the RandR event
RROutputChangeNotifyEvent and X11 applications relying on that event
like Qt will fail to emit their signal QGuiApplication::screenRemoved.

To avoid that issue, make sure to mark the RandR output as disconnected
prior to call xwl_output_remove().

Fix commit 204f10c2 ("xwayland: Call RRTellChanged if the RandR configuration may have changed")

Signed-off-by: zhoulei <zhoulei@kylinos.cn>
Signed-off-by: Morose <chenlinxiang@kylinos.cn>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>