winsys/amdgpu,radeonsi: add HUD counters for how much memory is wasted by slabs

winsys/amdgpu,radeonsi: add HUD counters for how much memory is wasted by slabs

Slabs always allocate the next power of two size from their pools. This
wastes memory if the size is not a power of two.

bo->base.size is overwritten because the default is the allocated power of
two size, but we need the real size to compute the wasted size in
amdgpu_bo_slab_destroy. entry_size is added to the hole in pb_slab_entry
to hold the real entry size.

Like other memory stats, no atomics are used.

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