radv: Add VEGA20 support.

Just mirror the radeonsi bits. Since this is just adding the extra
switch entries for new HW I think this should be fine for stable.

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 4dc244eb)