radv: Only allow 16 user SGPRs for compute on GFX9+.

Apparently for compute there are only 16 instead of the 32 for the
graphics path.

Fixes dEQP-VK.binding_model.descriptorset_random.sets16.noarray.ubolimitlow.sbolimitlow.imglimitlow.noiub.comp.0

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 0dd8189f)