drm/i915: nonblocking commit

Simply split intel_atomic_commit in half and place the new
nonblocking commit helpers at the right spots.

NOTE: There's still trouble with obj->frontbuffer bits getting mangled
when pipelining atomic commits.

v2:
- Remove the check for nonblocking which returned -EINVAL.
- Do wait for requests in the worker thread before committing
  hw state.

v3: Move hw_done after the optimize_wm/post_plane_update step, plus
add FIXME comment how to fix that up again properly.

v4: Update FIXME for intel_atomic_commit - more stuff works now.

v5: Still reject nonblocking modeset commits (Maarten).

v6: Use intel_state->modeset (Maarten).

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465920060-6388-1-git-send-email-daniel.vetter@ffwll.ch