drm/nouveau/fifo: turn chan subdev mask into engine mask

This data is used to know which engines/classes are reachable on a given
channel's runlist, and needs to be replaced with something that doesn't
rely on subdev index.

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