rust: drm: gem: s/into_gem_obj()/as_raw()/

There's a few changes here:
* The rename, of course (this should also let us drop the clippy annotation
  here)
* Return *mut bindings::drm_gem_object instead of
  &Opaque<bindings::drm_gem_object> - the latter doesn't really have any
  benefit and just results in conversion from the rust type to the C type
  having to be more verbose than necessary.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/r/20250513221046.903358-4-lyude@redhat.com


[ Fixup s/into_gem_obj()/as_raw()/ in safety comment. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>