libweston/drm-virtual: Point output base backend the DRM backend

This would allow output_repaint_timer_handler() to find a backend
as well for the DRM virtual outputs created by DRM virtual API and
with it to trigger a repaint for the outputs created by
plug-ins (remoting and pipewre).

Fixes 1f8c49d5bdd20, 'compositor: repaint backends separately'

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Michael Olbrich <m.olbrich@pengutronix.de>