drm: rcar-du: Clamp DPMS states to on and off

The intermediate DPMS standby and suspend states are a thing from the
past. They only matter in practice for VGA CRT monitors, and are just a
power saving vs. resume time optimization. Given that they have never
been implemented properly in the rcar-du driver and that the Intel
driver has dropped them on the vga port years ago, it's safe to only
care about the on and off states.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>