video_core/memory_manager: Make GpuToCpuAddress() a const member function

This doesn't modify any internal state, so it can be made a const member
function to allow its use in const contexts.

src/video_core/memory_manager.cpp

@@ -99,12 +99,12 @@ bool MemoryManager::IsAddressValid(GPUVAddr addr) const {
     return (addr >> page_bits) < page_table.pointers.size();
 }
 
-std::optional<VAddr> MemoryManager::GpuToCpuAddress(GPUVAddr addr) {
+std::optional<VAddr> MemoryManager::GpuToCpuAddress(GPUVAddr addr) const {
     if (!IsAddressValid(addr)) {
         return {};
     }
 
-    VAddr cpu_addr{page_table.backing_addr[addr >> page_bits]};
+    const VAddr cpu_addr{page_table.backing_addr[addr >> page_bits]};
     if (cpu_addr) {
         return cpu_addr + (addr & page_mask);
     }

src/video_core/memory_manager.h

@@ -50,7 +50,7 @@ public:
     GPUVAddr MapBufferEx(VAddr cpu_addr, u64 size);
     GPUVAddr MapBufferEx(VAddr cpu_addr, GPUVAddr addr, u64 size);
     GPUVAddr UnmapBuffer(GPUVAddr addr, u64 size);
-    std::optional<VAddr> GpuToCpuAddress(GPUVAddr addr);
+    std::optional<VAddr> GpuToCpuAddress(GPUVAddr addr) const;
 
     template <typename T>
     T Read(GPUVAddr addr);