nvdrv: Cleanup CDMA Processor on device closure

Brings us a step closer to unifying all channels to share a common interface.

src/video_core/gpu.h

@@ -324,6 +324,9 @@ public:
     /// Push GPU command buffer entries to be processed
     void PushCommandBuffer(Tegra::ChCommandHeaderList& entries);
 
+    /// Frees the CDMAPusher to free up resources
+    void ClearCommandBuffer();
+
     /// Swap buffers (render frame)
     void SwapBuffers(const Tegra::FramebufferConfig* framebuffer);