drm/vc4: tests: Use KUNIT_DEFINE_ACTION_WRAPPER

In order to pass functions to kunit_add_action(), they need to be of the
kunit_action_t type. While casting the function pointer can work, it
will break control-flow integrity.

vc4_mock already defines such a wrapper for drm_dev_unregister(), but it
involves less boilerplate to use the new macro, so replace the manual
implementation.

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>