rust: drm: gem: Use NonNull for Object::dev

There is usually not much of a reason to use a raw pointer in a data
struct, so move this to NonNull instead.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/r/20250513221046.903358-2-lyude@redhat.com


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