video_core/memory_manager: Make ReadBlock() a const qualifier member function

Now, since we have a const qualified variant of GetPointer(), we can put
it to use in ReadBlock() to retrieve the source pointer that is passed
into memcpy.

Now block reading may be done from a const context.

src/video_core/memory_manager.cpp

@@ -203,7 +203,7 @@ const u8* MemoryManager::GetPointer(GPUVAddr addr) const {
     return {};
 }
 
-void MemoryManager::ReadBlock(GPUVAddr src_addr, void* dest_buffer, std::size_t size) {
+void MemoryManager::ReadBlock(GPUVAddr src_addr, void* dest_buffer, std::size_t size) const {
     std::memcpy(dest_buffer, GetPointer(src_addr), size);
 }
 void MemoryManager::WriteBlock(GPUVAddr dest_addr, const void* src_buffer, std::size_t size) {

src/video_core/memory_manager.h

@@ -61,7 +61,7 @@ public:
     u8* GetPointer(GPUVAddr addr);
     const u8* GetPointer(GPUVAddr addr) const;
 
-    void ReadBlock(GPUVAddr src_addr, void* dest_buffer, std::size_t size);
+    void ReadBlock(GPUVAddr src_addr, void* dest_buffer, std::size_t size) const;
     void WriteBlock(GPUVAddr dest_addr, const void* src_buffer, std::size_t size);
     void CopyBlock(GPUVAddr dest_addr, GPUVAddr src_addr, std::size_t size);