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 e8cb455876fa8f67c6aba394d0a14b697bf04cc3
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>