drm/i915/gvt: Enable cmd_parser support for BXT.

Handle BXT cmd_parser as SKL/KBL.

v2: All supported platforms share the same routines.
    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>