drm/nouveau/subdev: track type+instance separately

We use subdev id bitmasks (as a u64) in a number of places, and GA100 adds
enough new engine instances that we run out of bits.  We could alias IDs of
engines that no longer exist, but it's cleaner for a number of reasons to
just split the subdev index into a subdev type, and instance ID instead.

Just a lot more painful to do.

This magics up the values for old-style subdev constructors, and provides a
way to incrementally transition each subdev to the new style.

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