drm_hwcomposer: C++ coding style cleanup, no functional changes.

Apply "override" keyword and remove redundant "virtual" keyword.
Ran clang-format on all files.

Change-Id: I76730296aa8f52e555137d4b5973fa27d38f0aa6
diff --git a/drmdisplaycompositor.h b/drmdisplaycompositor.h
index c1b4bcd..e756fde 100644
--- a/drmdisplaycompositor.h
+++ b/drmdisplaycompositor.h
@@ -108,14 +108,14 @@
   class FrameWorker : public Worker {
    public:
     FrameWorker(DrmDisplayCompositor *compositor);
-    ~FrameWorker();
+    ~FrameWorker() override;
 
     int Init();
     void QueueFrame(std::unique_ptr<DrmDisplayComposition> composition,
                     int status);
 
    protected:
-    virtual void Routine();
+    void Routine() override;
 
    private:
     DrmDisplayCompositor *compositor_;