video_core/engines: Remove unnecessary inclusions where applicable

Replaces header inclusions with forward declarations where applicable
and also removes unused headers within the cpp file. This reduces a few
more dependencies on core/memory.h

src/video_core/engines/maxwell_3d.h

@@ -16,13 +16,16 @@
 #include "common/math_util.h"
 #include "video_core/gpu.h"
 #include "video_core/macro_interpreter.h"
-#include "video_core/memory_manager.h"
 #include "video_core/textures/texture.h"
 
 namespace Core {
 class System;
 }
 
+namespace Tegra {
+class MemoryManager;
+}
+
 namespace VideoCore {
 class RasterizerInterface;
 }