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>