drm/i915: Enable KVMGT for BXT.

Enable KVMGT for BXT.
is_supported_device() acting as the gatekeeper of GVT-g init.
If all supported platforms share the same configurations for some
specific feature, platform check will rely on this check only.

Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>