drm_hwcomposer: Rework DrmMode class

Simplify code and raise-up clang-tidy level of DrmMode class
to 'normal'.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/drm/DrmConnector.h b/drm/DrmConnector.h
index 1e1a685..63150e7 100644
--- a/drm/DrmConnector.h
+++ b/drm/DrmConnector.h
@@ -123,7 +123,7 @@
 
   const uint32_t index_in_res_array_;
 
-  DrmMode active_mode_;
+  DrmMode active_mode_{};
   std::vector<DrmMode> modes_;
 
   DrmProperty dpms_property_;