drm/i915/gvt: let NOPID be the default value of force_to_nonpriv registers

Each ring has a NOPID register and currently they are regarded as default
value of force_to_nonpriv registers in guest drivers

Signed-off-by: Zhao Yan <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>