drm/vc4: hdmi: Disable Wifi Frequencies

There's cross-talk on the RPi4 between the 2.4GHz channels used by the WiFi
chip and some resolutions, most notably 1440p at 60Hz.

In such a case, we can either reject entirely the mode, or lower slightly
the pixel frequency to remove the overlap. Let's go for the latter.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201029134018.1948636-2-maxime@cerno.tech