Rework:
* Jordan: Handle per_thread_scratch==0 in anv_scratch_pool_get_surf
* Jordan: Update subslices in anv_scratch_pool_alloc
* Jason: Clean up the patch a bit
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11582>