drm/i915: add port field to struct intel_dp and use it

This will be needed for Haswell, but already has its uses here.

This patch started as a small patch written patch by Shobhit Kumar,
but it has changed so much that none of its original lines remain.

Credits-to: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>