drm/i915/dp: set sink to power down mode on dp disable
but for DDI, where we've never done this.
Signed-off-by: Jani Nikula <jani.nikula@intel.com> Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com> Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>