VideoCommon: Move logging/seen check for unknown opcodes into CommandProcessor

That way, they're in the same place the panic alerts are generated.

Source/Core/VideoCommon/VideoBackendBase.cpp

@@ -316,7 +316,6 @@ void VideoBackendBase::InitializeShared()
 
   CommandProcessor::Init();
   Fifo::Init();
-  OpcodeDecoder::Init();
   PixelEngine::Init();
   BPInit();
   VertexLoaderManager::Init();