gpu modesetting improvements

The first commit is just a bugfix, for when the old Probe method is used if a device is specified using the "kmsdevice" option. This previousely didn't work with multiple devices.

The other commits relax which device nodes can be used for what. There is a slight overlap with !469, but this merge request doesn't replace it, if anything, it may be a good addition to this MR.

For a more detailed explanation of what the commits do, please look at the individual commit messages.