drm/vc4: tests: Fail the current test if we access a register

Accessing a register when running under kunit is a bad idea since our
device is completely mocked.

Fail the current test if we ever access any of our hardware registers.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-18-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>