drm/atomic: Move drm_crtc_commit to drm_crtc_state, v4.

Most code only cares about the current commit or previous commit.
Fortuantely we already have a place to track those. Move it to
drm_crtc_state where it belongs. :)

The per-crtc commit_list is kept for places where we have to look
deeper than the current or previous commit for checking whether to stall
on unpin. This is used in drm_atomic_helper_setup_commit and
intel_has_pending_fb_unpin.

Changes since v1:
- Update kerneldoc for drm_crtc.commit_list. (danvet)
Changes since v2:
- Remove drm_atomic_helper_async_check hunk. (pinchartl)
Changes since v3:
- Fix use-after-free in drm_atomic_helper_commit_cleanup_done().

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170904150456.31049-1-maarten.lankhorst@linux.intel.com
[mlankhorst: preceeding -> preceding (checkpatch)]