drm/vc4: tests: mock: Use a kunit action to unregister DRM device

The *_mock_device functions allocate a DRM device that needs to be
released using drm_dev_unregister.

Now that we have a kunit release action API, we can switch to it and
don't require any kind of garbage collection from the caller.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Link: https://lore.kernel.org/r/20230728-kms-kunit-actions-rework-v3-8-952565ccccfe@kernel.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>