Starting with Haswell, DDI ports can work in FDI mode to support
connectivity with the outputs located on the PCH.

This commit adds support for such connections in the intel_ddi module, and
provides Haswell-specific functionality to make it work.

v2: simplify the commit as per Daniel Vetter suggestion.

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