drm/i915/dp: set sink to power down mode on dp disable

drm/i915/dp: set sink to power down mode on dp disable

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 3600e4421c15aa3d1dedf7a356cb84b30f34e712
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>