drm/i915: Read eDP Display control capability registers

Add new edp_dpcd variable to intel_dp.
Read and save eDP Display control capability registers to edp_dpcd.

Signed-off-by: Yetunde Adebisi <yetundex.adebisi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459865452-9138-3-git-send-email-yetundex.adebisi@intel.com