gma500: Add the support of display port on CDV

Import the pieces we need in order to do DisplayPort. Don't wire them
up yet as there is work to do to integrate them.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>