drm/vc4: tests: Introduce a mocking infrastructure

In order to test the current atomic_check hooks we need to have a DRM
device that has roughly the same capabilities and layout that the actual
hardware. We'll also need a bunch of functions to create arbitrary
atomic states.

Let's create some helpers to create a device that behaves like the real
one, and some helpers to maintain the atomic state we want to check.

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-v3-17-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>