As crocus won't support any of these (BDW+) they should go into iris.
This also allows us to remove the "prefer_iris" option, as iris is now
the only option
Reviewed-by: Emma Anholt <emma@anholt.net>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10153>