drm/vc4: hdmi: Set HDMI_MAI_FMT

The hardware uses this for generating the right audio
data island packets when using formats other than PCM

Signed-off-by: Dom Cobley <popcornmix@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210525132354.297468-8-maxime@cerno.tech