drm/atomic: export drm_atomic_helper_wait_for_fences()

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1458342904-23326-3-git-send-email-robdclark@gmail.com