drm/i915: Split intel_enable_ddi() into DP and HDMI variants

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

v2: Keep using intel_ddi_get_encoder_port() for now

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010121207.570-10-ville.syrjala@linux.intel.com


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