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