This is needed by drm_output_fini_egl() to be able to retrieve the
backend out of the drm_output on the shutdown path of the compositor.
Both the remoting plug-in and the pipewire plug-in are users of the
drm-virtual API and as such they would trigger a crash when shutting
down the compositor, as we're not setting up any backend whatsoever.
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
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