drm/i915/gvt: do not return error on handling force_to_nonpriv registers

Return error will cause vm hang and enter failsafe mode.
However, we don't want that happen on detecting an wrong force_to_nonpriv
register write.
Therefore, we just omit the wrong write or patch it to default value.

v2: only return 0 on detecting lri write of registers outside whitelist,
but still return error on other error conditions.  (zhenyu wang)

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