WIP backend-drm: set connector max bpc

When enabling HDR or WCG modes, 8 bits per channel is not enough. Try to
push the cable as high as it can, up to 16.

Theoretically, this should avoid seeing banding when HDR mode is
correctly programmed, but I'm not that far yet.

This also makes SDR mode to explicitly program max bpc to 8. That's what
one would expect to use, so might as well make that, rather than rely on
luck.

TODO: wayland/weston#612



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>