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

Similar to
commit fdbc3b1f
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Tue Nov 12 17:10:13 2013 +0200

    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>