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/virtualcompositorworker.h b/virtualcompositorworker.h
index 4a15f0c..3066547 100644
--- a/virtualcompositorworker.h
+++ b/virtualcompositorworker.h
@@ -27,13 +27,13 @@
 class VirtualCompositorWorker : public Worker {
  public:
   VirtualCompositorWorker();
-  ~VirtualCompositorWorker();
+  ~VirtualCompositorWorker() override;
 
   int Init();
   void QueueComposite(hwc_display_contents_1_t *dc);
 
  protected:
-  virtual void Routine();
+  void Routine() override;
 
  private:
   struct VirtualComposition {