drm/vc4: kms: Warn if we have an incompatible muxing setup

The documentation explicitly states we must prevent the output
2 and 3 from feeding from the same HVS channel.

Let's add a warning to make some noise if we ever find ourselves in such
a case.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20220328153659.2382206-6-maxime@cerno.tech