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/drmhwcgralloc.h b/drmhwcgralloc.h
index 880c8a4..765c897 100644
--- a/drmhwcgralloc.h
+++ b/drmhwcgralloc.h
@@ -64,4 +64,4 @@
   void *priv;
 } hwc_drm_bo_t;
 
-#endif // ANDROID_DRMHWCGRALLOC_H_
+#endif  // ANDROID_DRMHWCGRALLOC_H_