drm/vc4: hdmi: Register HDMI codec

The hdmi-codec brings a lot of advanced features, including the HDMI
channel mapping. Let's use it in our driver instead of our own codec.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210525132354.297468-11-maxime@cerno.tech