drm/atomic: Use new iterator macros in drm_atomic_helper_wait_for_flip_done, again.

drm/atomic: Use new iterator macros in drm_atomic_helper_wait_for_flip_done, again.

for_each_obj_in_state is about to be removed, so use the correct new
iterator macro.

I renamed the variable to 'unused', but forgot to convert
drm_atomic_helper_wait_for_flip_done to the new iterator macro,
so make it work this time.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20170719143920.25685-2-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>