drm/i915: add DP support to intel_ddi_disable_port

Just a missing register. There is no problem to run this code when the
output is HDMI.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>