drm: nova: update ARef import from sync::aref

drm: nova: update ARef import from sync::aref

Update nova to import `ARef` from `sync::aref` instead of `types`.

This aligns with the ongoing effort to move `ARef` and
`AlwaysRefCounted` to sync.

Suggested-by: Benno Lossin <lossin@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1173
Signed-off-by: Shankari Anand <shankari.ak0208@gmail.com>
Link: https://lore.kernel.org/r/20250716090941.811418-1-shankari.ak0208@gmail.com
[ Alter subject and commit message to be nova specific. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>