vkd3d: Do not store the latch bit in an object that could be overwritten.

Once a event is signaled, the corresponding struct vkd3d_waiting_event entry is considered dead and could be overwritten, so it's not safe to keep a pointer to it in d3d12_fence_SetEventOnCompletion(). Instead, keep the latch bit in d3d12_fence_SetEventOnCompletion() and put a pointer to it in struct vkd3d_waiting_event.