drm/i915/gvt: fix an error for F_RO flag

the ro_mask is not stored into each mmio entry

Fixes: 12d14cc4

 ("drm/i915/gvt: Introduce a framework for tracking HW registers.")
Signed-off-by: Zhao Yan <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>