hwcontext_vaapi: Only accept a render node when deriving from DRM device

hwcontext_vaapi: Only accept a render node when deriving from DRM device

If we are given a non-render node, try to find the matching render node and
fail if that isn't possible.

libva will not accept a non-render device which is not DRM master, because
it requires legacy DRM authentication to succeed in that case:
<https://github.com/intel/libva/blob/master/va/drm/va_drm.c#L68-L75>.  This
is annoying for kmsgrab because in most recording situations DRM master is
already held by something else (such as a windowing system), leading to
device derivation not working and forcing the user to create the target
VAAPI device separately.