drm/atomic: Remove waits in drm_atomic_helper_commit_cleanup_done, v2.

When commit synchronization through drm_crtc_commit was first
introduced, we tried to solve the problem of the flip_done
needing a reference count by blocking in cleanup_done.

This has been changed by commit 24835e44 ("drm: reference count
event->completion") which made the waits here no longer needed.

However, even after this commit we still needed the wait because
otherwise we cannot wait for the flip_done because this item might
have been removed from the list.

Changed since v1:
- Make mention of cleanup_done completing before flip_done.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170904104838.23822-4-maarten.lankhorst@linux.intel.com