gpu: nova-core: remove `io::` qualifier to register macro invocations

gpu: nova-core: remove `io::` qualifier to register macro invocations

The kernel's `register` macro would clash with nova-core's own version
if it was imported directly, so it was accessed through its `io` module
during the conversion phase.

Now that nova-core's `register` macro doesn't exist anymore, we can
import and use it directly without risk of name collision.

Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260325-b4-nova-register-v4-9-bdf172f0f6ca@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>