drm/i915: add DDI registers

There is one set of such registers for each pipe (A/B/C/EDP).

v2: update to use DDI PORTS enum

v1 Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>