gpu: nova-core: register: move OFFSET declaration to I/O impl block

gpu: nova-core: register: move OFFSET declaration to I/O impl block

The OFFSET const is an I/O property, and having to pass it to the
@common rule makes it impossible to make I/O optional, as we want to get
to eventually.

Thus, move OFFSET to the I/O impl block so it is not needed by the
@common rule anymore.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250718-nova-regs-v2-7-7b6a762aa1cd@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>