drm/vc4: Add comments for which HVS_PIXEL_ORDER_xxx defines apply

The HVS_PIXEL_ORDER_xxx defines apply to specific HVS_PIXEL_FORMAT_xxx
modes, so add comments to make this obvious.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20221207-rpi-hvs-crtc-misc-v1-12-1f8e0770798b@cerno.tech


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