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/drmcompositorworker.h b/drmcompositorworker.h
index 9f74fb6..00e14ee 100644
--- a/drmcompositorworker.h
+++ b/drmcompositorworker.h
@@ -26,12 +26,12 @@
 class DrmCompositorWorker : public Worker {
  public:
   DrmCompositorWorker(DrmDisplayCompositor *compositor);
-  ~DrmCompositorWorker();
+  ~DrmCompositorWorker() override;
 
   int Init();
 
  protected:
-  virtual void Routine();
+  void Routine() override;
 
   DrmDisplayCompositor *compositor_;
 };