drm: nova: convert to use DRM registration data

Move the auxiliary device reference from drm::Device data into
RegistrationData, replacing the ARef<auxiliary::Device> with a borrowed
&'bound auxiliary::Device<Bound>. This makes the data lifetime-aware and
exercises the registration data path through the ioctl handlers.

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