udev + gpu screens + output slaves

[Dave Airlie]

Okay this is feature set 1, output slaves, it contains the
previous set of infrastructure changes, cleaned up further
and some more little bugs fixed.

Changes that need definite thinking about are mainly the
privates assert removals.

But after this set, you get to attach output providers,
and have their crtc/outputs appear. Of course you need
drivers that support the hooks to make all the magic happen.

Dave.