drm/vc4: tests: Add unit test suite for the PV muxing

The HVS to PixelValve muxing code is fairly error prone and has a bunch
of arbitrary constraints due to the hardware setup.

Let's create a test suite that makes sure that the possible combinations
work and the invalid ones don't.

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-v3-19-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>