drm/vc4: hdmi: Enable the scrambler

The HDMI controller on the BCM2711 includes a scrambler in order to
reach the HDMI 2.0 modes that require it. Let's add the support for it.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210507150515.257424-11-maxime@cerno.tech