drm/nouveau/fifo: add chid allocator

We need to be able to allocate TSG IDs as well as channel IDs, also,
Ampere has per-runlist channel IDs.

- holds per-ID private data, which will be used for/to protect lookup
- holds an nvkm_event which will be used for events tied to IDs
- not used yet beyond setup, and switching use of "fifo->nr - 1" for
  channel ID mask to "chid->mask"

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>