backend-drm: set connector max bpc

"max bpc" property is meant for working around faulty sink hardware.
Normally it should be set to the maximum possible value so that the
kernel driver has full freedom to choose the link bpc without being
artificially forced to lower color precision.

The default value is 16 because that is a nice round number and more
than any link technology I've heard is using today which would be 12.

Also offer an API set the value, so that weston.ini could be used in the
next patch for sink workaround purposes.

Closes: #612



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