drm/i915/gvt: add vreg write for GDRST handler

The emulation handler for MMIO GDRST miss vreg write in it, as result
the vreg cannot update correspondingly.

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