drm/i915/gvt: emulate right behavior for tlb_control

Emulate right behavior for tlb_control, set to ZERO upon write.

Signed-off-by: Ping Gao <ping.a.gao@intel.com>
Signed-off-by: Min He <min.he@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>