drm/nouveau/fifo: add cgrp, have all channels be part of one

Engine context tracking will move to nvkm_cgrp in later commits, so we
create SW-only channel groups on HW without support for them.

- switches to nvkm_chid for TSG/channel ID allocation

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

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>