drm/vc4: hdmi: Compute the CEC clock divider from the clock rate

The CEC clock divider needs to output a frequency of 40kHz from the HSM
rate on the BCM2835. The driver used to have a fixed frequency for it,
but that changed for the BCM2711 and we now need to compute it
dynamically to maintain the proper rate.

Fixes: cd4cb49d ("drm/vc4: hdmi: Adjust HSM clock rate depending on pixel rate")
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>