misc prime/modesetting fixes

[Dave Airlie]

The first patch is picked from Alex Goins' series as a cleanup,
I then got distracted fixing other minor things I found just testing
prime with modesetting.

Finally Alex's series showed up a bug we have currently leaking
the slave BO, his fix wasn't very symmetrical, however to make a
nice fix I had to break the ABI by passing a NULL where we didn't
before.

Dave.