drm/vc4: hdmi: Support HDMI YUV output

In addition to the RGB444 output, the BCM2711 HDMI controller supports
the YUV444 and YUV422 output formats.

Let's add support for them in the driver, but still use RGB as the
preferred format.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>