drm/i915/ddi: Move DDI port detection to the corresponding helper

drm/i915/ddi: Move DDI port detection to the corresponding helper

We have already a function to detect DDI ports using VBT, so instead of
opencoding the DDI specific version of this, move the opencoded part to
the existing helper.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181220132604.25222-1-imre.deak@intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>