drm/i915/gvt: Extend KBL platform support in GVT-g

Extend KBL platform support in GVT-g. Validation tests
are done on KBL server and KBL NUC. Both show the same
quality.

Signed-off-by: Jian Jun Chen <jian.jun.chen@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>