drm/vc4: hdmi: Enable 10/12 bpc output

The BCM2711 supports higher bpc count than just 8, so let's support it in
our driver.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215154243.540115-10-maxime@cerno.tech