gpu hotplug stack refresh

[Dave Airlie]

This is a refresh send of the whole gpu screen + offload slave + output
slave stack.

I'd really like to send a pull req for this, but I'm striving to find any more
reviews of it. Aaron might get some more of it looked over hopefully, and I'll
kick ajax again to look at it.

I really want people to look at the new interfaces introduced in the screenrec,
and any APIs this introduces. (I'll send another email about best methods to workaround ABI freezes).

I think the pixmap sharing stuff works fine at this level, but at the moment
I'm not passing any tiling info between drivers, though now the drivers can
access each other ScreenRecs, so maybe I just need some specific hooks without
making any changes to the APIs.

Dave.