video_core/dma_pusher: The full list of headers at once.

Fetching every u32 from memory leads to a big overhead. So let's fetch all of them as a block if possible.
This reduces the Memory::* calls by the dma_pusher by a factor of 10.

src/video_core/dma_pusher.h

@@ -75,6 +75,8 @@ private:
 
     GPU& gpu;
 
+    std::vector<CommandHeader> command_headers; ///< Buffer for list of commands fetched at once
+
     std::queue<CommandList> dma_pushbuffer; ///< Queue of command lists to be processed
     std::size_t dma_pushbuffer_subindex{};  ///< Index within a command list within the pushbuffer