[dxvk] Properly clear used command buffer flags

Fixes an issue where we would always submit the init buffer, even
if it is empty.

src/dxvk/dxvk_cmdlist.cpp

@@ -109,7 +109,7 @@ namespace dxvk {
     
     // Unconditionally mark the exec buffer as used. There
     // is virtually no use case where this isn't correct.
-    m_cmdBuffersUsed.set(DxvkCmdBuffer::ExecBuffer);
+    m_cmdBuffersUsed = DxvkCmdBuffer::ExecBuffer;
   }