drm/vc4: hdmi: Limit the BCM2711 to the max without scrambling

Unlike the previous generations, the HSM clock limitation is way above
what we can reach without scrambling, so let's move the maximum
frequency we support to the maximum clock frequency without scrambling.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215154243.540115-9-maxime@cerno.tech