gallium/pipebuffer: add pb_slab utility

gallium/pipebuffer: add pb_slab utility

This is a simple framework for slab allocation from buffers that fits into
the buffer management scheme of the radeon and amdgpu winsyses where bufmgrs
aren't used.

The utility knows about different sized allocations and explicitly manages
reclaim of allocations that have pending fences. It manages all the free lists
but does not actually touch buffer objects directly, relying on callbacks for
that.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>