drm/i915: add enumeration for DDI ports

There are 5 DDI ports on Haswell. Port A is always enabled, and is the one
connected to eDP, and Port E is the one that can be connected to the PCH
using FDI protocol.  Ports B, C, D and E can be used for digital outputs.

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