We used to put the local sink and any downstream sinks to power down
mode at disable or dpms off using the DPCD SET_POWER register, until
this was broken by

commit e8cb4558
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Jul 1 13:05:48 2012 +0200

    drm/i915/dp: convert to encoder disable/enable

Fix it.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>