Currently we neatly track the crtc state, but forget to look at
plane/connector state.

When doing a nonblocking modeset, immediately followed by a setprop
before the modeset completes, the setprop will see the modesets new
state as the old state and free it.

This has to be solved by waiting for hw_done on the connector, even
if it's not assigned to a crtc. When a connector is unbound we take
the last crtc commit, and when it stays unbound we create a new
fake crtc commit for that gets signaled on hw_done for all the
planes/connectors.

We wait for it the same way as we do for crtc's, which will make
sure we never run into a use-after-free situation.

Changes since v1:
- Only create a single disable commit. (danvet)
- Fix leak in intel_legacy_cursor_update.
Changes since v2:
- Make reference counting in drm_atomic_helper_setup_commit
  more obvious. (pinchartl)
- Call cleanup_done for fake commit. (danvet)
- Add comments to drm_atomic_helper_setup_commit. (danvet, pinchartl)
- Add comment to drm_atomic_helper_swap_state. (pinchartl)
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Testcase: kms_atomic_transition.plane-use-after-nonblocking-unbind*
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170904104838.23822-6-maarten.lankhorst@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>