Moving LRU handling into Nouveau v2

Hi guys,

I already tried this a few month ago, but since I don't have NVidia hardware its rather hard to test for me (need to get some ordered).

Dave brought up the topic that we should probably try to move the handling into Nouveau once more, so I tried to fix the problem Ben reported and rebased on top of current drm-misc-next.

Dave can you test this? At least in theory the approach should work, but in practice there can of course be bugs.

Thanks, Christian.