radv: refactor descriptor set layout tracking

Separately track descriptor set layouts for pools with
FREE_DESCRIPTOR_SET disabled, instead of reusing
`radv_descriptor_pool::entries`. This saves 8 bytes for each entry and
also avoids some indirection.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16541>