service/nvflinger: Move display specifics over to vi_display

With the display and layer structures relocated to the vi service, we
can begin giving these a proper interface before beginning to properly
support the display types.

This converts the display struct into a class and provides it with the
necessary functions to preserve behavior within the NVFlinger class.

src/core/hle/service/nvflinger/nvflinger.h

@@ -28,7 +28,7 @@ class Module;
 } // namespace Service::Nvidia
 
 namespace Service::VI {
-struct Display;
+class Display;
 struct Layer;
 } // namespace Service::VI