drm/i915: Split intel_disable_ddi() into DP vs. HDMI variants

Untangle intel_disable_ddi() by splitting it into DP and HDMI specific
variants.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010121207.570-8-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>