drm/vc4: hdmi: Store pixel frequency in the connector state

The pixel rate is for now quite simple to compute, but with more features
(30 and 36 bits output, YUV output, etc.) will depend on a bunch of
connectors properties.

Let's store the rate we have to run the pixel clock at in our custom
connector state, and compute it in atomic_check.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215154243.540115-7-maxime@cerno.tech