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 enough for today's technology.

Also offer an API to set the value, so that weston.ini can be used to set the workaround.

Closes: #612 (closed)

cc @swick