drm/i915/bdw: check DPD on port D when setting the DDI buffers

Use the eDP values on platforms where port D is eDP. This doesn't
affect Haswell since it uses the same DDI buffer values for eDP and
DP.

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