drm/i915: add support for DDI-controlled digital outputs

Those are driven by DDIs on Haswell architecture, so we need to keep track
of which DDI is being used on each output.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>