drm/i915/gvt: Fix GDRST vreg state after reset

Emulating the GDRST read behavior correctly to ack the
guest reset request.

v2:
- split the original patch into two:
  GDRST read handler and virtual gpu reset. (Zhenyu)
v3:
- emulate the GDRST read right after write. (Zhenyu)

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhang Yulei <yulei.zhang@intel.com>
Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>