drm/i915/gvt: opregion virtualization for win guest

this is an enhanced opregion emulation for win guest support
by initializing more data members including opregion header
size, version and child device propertity for display port.
for simplicity, redefined child_device_config structure.

Signed-off-by: Xiaolin Zhang <xiaolin.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>