drm/nouveau/fifo: add new engine context tracking

Channel groups have somewhat more complicated requirements than what we
currently support.  An engine context is shared between all channels in
a channel group, VEID/subctx support (later) brings per-VEID components,
and we need to track an individual channel's engine context pointers.

This commit adds the structures and refcounting to support the above,
wrapping the prior implementation for the moment.

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