gpu: Make memory_manager private

Makes the class interface consistent and provides accessors for
obtaining a reference to the memory manager instance.

Given we also return references, this makes our more flimsy uses of
const apparent, given const doesn't propagate through pointers in the
way one would typically expect. This makes our mutable state more
apparent in some places.

src/video_core/gpu.cpp

@@ -22,7 +22,7 @@ u32 FramebufferConfig::BytesPerPixel(PixelFormat format) {
 }
 
 GPU::GPU(VideoCore::RasterizerInterface& rasterizer) {
-    memory_manager = std::make_unique<MemoryManager>();
+    memory_manager = std::make_unique<Tegra::MemoryManager>();
     maxwell_3d = std::make_unique<Engines::Maxwell3D>(rasterizer, *memory_manager);
     fermi_2d = std::make_unique<Engines::Fermi2D>(*memory_manager);
     maxwell_compute = std::make_unique<Engines::MaxwellCompute>();
@@ -31,12 +31,20 @@ GPU::GPU(VideoCore::RasterizerInterface& rasterizer) {
 
 GPU::~GPU() = default;
 
+Engines::Maxwell3D& GPU::Maxwell3D() {
+    return *maxwell_3d;
+}
+
 const Engines::Maxwell3D& GPU::Maxwell3D() const {
     return *maxwell_3d;
 }
 
-Engines::Maxwell3D& GPU::Maxwell3D() {
-    return *maxwell_3d;
+MemoryManager& GPU::MemoryManager() {
+    return *memory_manager;
+}
+
+const MemoryManager& GPU::MemoryManager() const {
+    return *memory_manager;
 }
 
 u32 RenderTargetBytesPerPixel(RenderTargetFormat format) {