drm/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.

drm_kunit_helpers already defines wrappers, but we now have a macro
which does this automatically. Using this greatly reduces the
boilerplate needed.

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