radv: Use a lower max offchip buffer count.

No clue what gets fixed by this but both radeonsi and amdvlk do it.

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 ab64891f)