meson: Remove panfrost from default driver list

Until the kernel side matures and the full driver is upstreamed, to
avoid end-user surprises, Panfrost should only be built for the
adventurous.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Eric Anholt <eric@anholt.net>
1 file changed