GPU: Flush commands on every dma pusher step.

This commit ensures that the host gpu is constantly fed with commands to
work with, while the guest gpu keeps producing the rest of the commands.
This reduces syncing time between host and guest gpu.

src/video_core/rasterizer_interface.h

@@ -50,6 +50,9 @@ public:
     /// and invalidated
     virtual void FlushAndInvalidateRegion(CacheAddr addr, u64 size) = 0;
 
+    // Notify the rasterizer to send all written commands to the host GPU.
+    virtual void FlushCommands() = 0;
+
     /// Notify rasterizer that a frame is about to finish
     virtual void TickFrame() = 0;