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/vsyncworker.h b/vsyncworker.h
index ce7b94a..98ac546 100644
--- a/vsyncworker.h
+++ b/vsyncworker.h
@@ -31,7 +31,7 @@
 class VSyncWorker : public Worker {
  public:
   VSyncWorker();
-  ~VSyncWorker();
+  ~VSyncWorker() override;
 
   int Init(DrmResources *drm, int display);
   int SetProcs(hwc_procs_t const *procs);
@@ -39,7 +39,7 @@
   int VSyncControl(bool enabled);
 
  protected:
-  virtual void Routine();
+  void Routine() override;
 
  private:
   int64_t GetPhasedVSync(int64_t frame_ns, int64_t current);