This is part of what atomic must implement. And it's also required
to be able to use the helper nonblocking support.

v2: Always send out the drm event, remove the planes_changed check.
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465827229-1704-1-git-send-email-daniel.vetter@ffwll.ch