There can be more than one available render node, and it's not
guaranteed the first node we come across is the correct one. In
particular, 'vgem' devices are common, and are
never VAAPI-enabled and thus not valid here.

We have a 'kernel_driver' arg already for specifying a single driver we
*do* want, but it doesn't support a negation, nor a list. It's easier
just to automatically skip 'vgem' anyway, to avoid foisting this burden
on users.

This has precedent in libva-utils already:

  bfb6b98ed62a exclude vgem node and invalid drm node in vainfo
  https://github.com/intel/libva-utils/commit/bfb6b98ed62ac14a840ba62639ab902a23912258Signed-off-by: Brian Norris <briannorris@chromium.org>