gpu: nova-core: register auxiliary device for nova-drm

Register an auxiliary device for nova-drm.

For now always use zero for the auxiliary device's ID; we don't use it
yet anyways. However, once it lands, we should switch to XArray.

Acked-by: Dave Airlie <airlied@redhat.com>
Link: https://lore.kernel.org/r/20250424160452.8070-2-dakr@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>