video_core/engines: Make memory manager members private

These aren't used externally by anything, so they can be made private
data members.

src/video_core/engines/maxwell_dma.h

@@ -142,13 +142,13 @@ public:
         };
     } regs{};
 
-    MemoryManager& memory_manager;
-
 private:
     Core::System& system;
 
     VideoCore::RasterizerInterface& rasterizer;
 
+    MemoryManager& memory_manager;
+
     /// Performs the copy from the source buffer to the destination buffer as configured in the
     /// registers.
     void HandleCopy();