radeonsi: rename and rearrange RW buffer slots

radeonsi: rename and rearrange RW buffer slots

- use an enum
- use a unique slot number regardless of the shader stage
  (the per-stage slots will go away for RW buffers)

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>