drm/vc4: kms: Improve logging

When debugging, finding out what muxing decisions were made and what the
actual core clock rate is is always useful, so let's add some more
messages.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20220328153659.2382206-7-maxime@cerno.tech