drm: Nuke drm_atomic_legacy_backoff

Finally all users are gone!

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170725080122.20548-9-daniel.vetter@ffwll.ch