hwcontext_vaapi: Don't require a render node when deriving from DRM

hwcontext_vaapi: Don't require a render node when deriving from DRM

The V4L2 driver does not actually have an associated DRM device at all, so
users work around the requirement by giving libva an unrelated display-only
device instead (which is fine, because it doesn't actually do anything with
that device).  This was broken by bc9b6358fb7315c0173de322472641766f6289da
forcing a render node, because the display-only device did not have an
associated render node to use.  Fix that by just passing through the
original non-render DRM fd if we can't find a render node.

Reported-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Tested-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>