drm/i915/gvt: Enable gtt initialization for BXT.

Initialize BXT gtt as SKL/KBL.

v2: All supported platforms share the same gtt ops.
    Remove the platform check by now and let is_supported_device()
    be the gate keeper.

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