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/nvimporter.h b/nvimporter.h
index a07f577..d6033db 100644
--- a/nvimporter.h
+++ b/nvimporter.h
@@ -29,12 +29,12 @@
 class NvImporter : public Importer {
  public:
   NvImporter(DrmResources *drm);
-  virtual ~NvImporter();
+  ~NvImporter() override;
 
   int Init();
 
-  virtual int ImportBuffer(buffer_handle_t handle, hwc_drm_bo_t *bo);
-  virtual int ReleaseBuffer(hwc_drm_bo_t *bo);
+  int ImportBuffer(buffer_handle_t handle, hwc_drm_bo_t *bo) override;
+  int ReleaseBuffer(hwc_drm_bo_t *bo) override;
 
  private:
   typedef struct NvBuffer {