rust: drm: Add gem::impl_aref_for_gem_obj!

rust: drm: Add gem::impl_aref_for_gem_obj!

In the future we're going to be introducing more GEM object types in rust
then just gem::Object<T>. Since all types of GEM objects have refcounting,
let's introduce a macro that we can use in the gem crate in order to copy
this boilerplate implementation for each type: impl_aref_for_gem_obj!().

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Janne Grunau <j@jananu.net>
Tested-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Link: https://patch.msgid.link/20260316211646.650074-2-lyude@redhat.com
[ Resolve merge conflicts. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>