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/video_core/rasterizer_interface.h

@@ -38,6 +38,9 @@ public:
     /// Notify rasterizer that any caches of the specified region should be flushed to 3DS memory
     virtual void FlushRegion(PAddr addr, u32 size) = 0;
 
+    /// Notify rasterizer that any caches of the specified region should be invalidated
+    virtual void InvalidateRegion(PAddr addr, u32 size) = 0;
+
     /// Notify rasterizer that any caches of the specified region should be flushed to 3DS memory
     /// and invalidated
     virtual void FlushAndInvalidateRegion(PAddr addr, u32 size) = 0;