Memory: Remove count of cached pages and add InvalidateRegion

In a future commit, the count of cached pages will be reintroduced in
the actual surface cache. Also adds an Invalidate only to the cache
which marks a region as invalid in order to try to avoid a costly flush
from 3ds memory

src/core/hle/kernel/vm_manager.cpp

@@ -58,7 +58,6 @@ void VMManager::Reset() {
 
     page_table.pointers.fill(nullptr);
     page_table.attributes.fill(Memory::PageType::Unmapped);
-    page_table.cached_res_count.fill(0);
 
     UpdatePageTableForVMA(initial_vma);
 }

src/core/hle/service/gsp/gsp_gpu.cpp

@@ -423,7 +423,7 @@ static void ExecuteCommand(const Command& command, u32 thread_id) {
                                              command.dma_request.size, Memory::FlushMode::Flush);
         Memory::RasterizerFlushVirtualRegion(command.dma_request.dest_address,
                                              command.dma_request.size,
-                                             Memory::FlushMode::FlushAndInvalidate);
+                                             Memory::FlushMode::Invalidate);
 
         // TODO(Subv): These memory accesses should not go through the application's memory mapping.
         // They should go through the GSP module's memory mapping.