drm/i915/bdw: add BDW DDI buf translations for eDP

Broadwell has different DDI buffer translations for eDP and DP, so add
support for the missing eDP and keep Haswell the same.

A future patch addresses the suggestion from Art to check for eDP on
port D and use the eDP values there, too.

v2: Make checkpatch happy.

Reviewed-by: Art Runyan <arthur.j.runyan@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v1)
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>