drm/i915: Add the support of eDP on DP-D for Ibex/CPT

This one adds support for eDP that connected on PCH DP-D port
instead of CPU DP-A port, and only DP-D port could be used for eDP.

https://bugs.freedesktop.org/show_bug.cgi?id=27220



Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Tested-by: Jan-Hendrik Zab <jan@jhz.name>
Tested-by: Templar <templar@rshc.de>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>