drm/nouveau/fifo: unify handling of channel classes

Adds the basic skeleton for common channel (group) interfaces.

- common behaviour between <gk104 and >=gk104 impl's
- separates priv/user channel objects
- passthrough to existing object for now, kludges removed later

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