drm/vc4: tests: pv-muxing: Remove call to drm_kunit_helper_free_device()

Calling drm_kunit_helper_free_device() to clean up the resources
allocated by drm_kunit_helper_alloc_device() is now optional and not
needed in most cases.

Remove it.

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-v3-7-952565ccccfe@kernel.org


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