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Design ideas:

- split up the actual commit into different phases, and have
  completions for each of them. This will be useful for the future
  when we want to interleave phases much more aggressively, for e.g.
  queue depth > 1. For not it's just a minimal optimization compared
  to current common nonblocking implementation patterns from drivers,
  which all stall for the entire commit to complete, including vblank
  waits and cleanups.

- Extract a separate atomic_commit_hw hook since that's the part most
  drivers will need to overwrite, hopefully allowing even more shared
  code.

- Enforce EBUSY seamntics by attaching one of the completions to the
  flip_done vblank event. Side benefit of forcing atomic drivers using
  these helpers to implement event handlign at least semi-correct. I'm
  evil that way ;-)

- Ridiculously modular, as usual.

- The main tracking unit for a commit stays struct drm_atomic_state,
  and the ownership rules for that are unchan...