drm/vc4: tests: Switch to atomic state allocation helper

Now that we have a helper that takes care of an atomic state allocation
and cleanup, we can migrate to it to simplify our tests.

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-v3-10-952565ccccfe@kernel.org


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