winsys/amdgpu,pb_slab: add slabs with 3/4 of power of two sizes to save memory

winsys/amdgpu,pb_slab: add slabs with 3/4 of power of two sizes to save memory

Instead of aligning slab allocations to powers of two (e.g. 129K -> 256K),
implement slab allocations with 3/4 of power of two sizes to reduce
overallocation. (e.g. 129K -> 192K)

The limitation is that the alignment must be 1/3rd of the allocation size.

DeusExMD allocates 2.1 GB of VRAM. Without this, slabs waste 194 MB due
to alignment, i.e. 9.2%. This commit reduces the waste to 102 MB, i.e. 4.9%.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8683>