v3dv/pipeline: reduce descriptor_map size

64 was a temporary and conservative "big enough" value, but we can do
better.

Note that as mentioned on the FIXME, we could be even more detailed,
adding a descriptor map allocate method based on the descriptor
type. That would mean more individual allocations, and slightly more
complexity.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10207>
1 file changed