drm/i915: Add eDP support on IGDNG mobile chip

This adds embedded DisplayPort support on next mobile chip which
aims to replace origin LVDS port. VBT's driver feature block has
been used to determine the type of current internal panel for eDP
or LVDS.

Currently no panel fitting support for eDP and backlight control
would be added in future.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>