drm/i915: Always wait for flip_done, v2.

The next commit removes the wait for flip_done in in
drm_atomic_helper_commit_cleanup_done, but we need it for the tests
to pass. Instead of using complicated vblank tracking which ends
up being ignored anyway, call the correct atomic helper. :)

Changes since v1:
- Always call drm_atomic_helper_wait_for_flip_done,
  even for legacy cursor updates. (danvet)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170904104838.23822-2-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>