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 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>