drm/vc4: hdmi: Create a custom connector state

When run with a higher bpc than 8, the clock of the HDMI controller needs
to be adjusted. Let's create a connector state that will be used at
atomic_check and atomic_enable to compute and store the clock rate
associated to the state.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215154243.540115-6-maxime@cerno.tech