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>

drivers/gpu/drm/i915/gvt/handlers.c

@@ -298,6 +298,9 @@ static int gdrst_mmio_write(struct intel_vgpu *vgpu, unsigned int offset,
 
 	intel_gvt_reset_vgpu_locked(vgpu, false, engine_mask);
 
+	/* sw will wait for the device to ack the reset request */
+	 vgpu_vreg(vgpu, offset) = 0;
+
 	return 0;
 }