vfio/igd: support legacy mode for all known generations

vfio/igd: support legacy mode for all known generations

We're soon going to add support for legacy mode to ElkhartLake and
TigerLake devices. Those are gen 11 and 12 devices. At the moment, all
devices identified by our igd_gen function do support legacy mode. This
won't change when adding our new devices of gen 11 and 12. Therefore, it
makes more sense to accept legacy mode for all known devices instead of
maintaining a long list of known good generations. If we add a new
generation to igd_gen which doesn't support legacy mode for some reason,
it'll be easy to advance the check to reject legacy mode for this
specific generation.

Signed-off-by: Corvin Köhne <c.koehne@beckhoff.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>