VideoCommon/CommandProcessor: Pass system instance through constructor

Makes the use of the interface a little less noisy, especially given
how much of the interface depends on an instance being present.

Source/Core/VideoCommon/OpcodeDecoding.cpp

@@ -220,7 +220,7 @@ public:
     else
     {
       auto& system = Core::System::GetInstance();
-      system.GetCommandProcessor().HandleUnknownOpcode(system, opcode, data, is_preprocess);
+      system.GetCommandProcessor().HandleUnknownOpcode(opcode, data, is_preprocess);
       m_cycles += 1;
     }
   }