drm/i915/gvt: Reuse the gmbus pin macro

Reuse the gmbus pin macro from i915_reg.h file to improve readablity.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>