[Intel-gfx] [RFC] async CRTC enable/disable hack

[Jesse Barnes]

Obviously still need a lot of work (and I didn't quite get the patch
split correctly, I meant commit the first bits earlier).

I think the approach may be sound though, and is actually not too hard
to get right with all the cross checking we have in place.  Things to
fix:
  - modeset cross check - we don't want to sync after a mode set just to
    check, maybe we could put this off until the actual enable happens
  - split flushing of disable and enable to actually make full mode sets
    fast

Anyway this is just a sketch of something we might do, definitely not
intended for upstream anytime soon.

Thanks,
Jesse