[dxvk] Only decrement pending submission count for command submissions

Otherwise we'll underflow the integer and break the D3D9 flush heuristic.

src/dxvk/dxvk_queue.cpp

@@ -215,6 +215,8 @@ namespace dxvk {
         entry.submit.cmdList->notifyObjects();
 
       lock.lock();
+
+      if (entry.submit.cmdList != nullptr)
         m_pending -= 1;
 
       m_finishQueue.pop();