drm_hwcomposer: CI: Raise clang-tidy level from NONE to COARSE for *.h

To improve CI coverage.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
diff --git a/compositor/Planner.h b/compositor/Planner.h
index ff059f5..c148b6e 100644
--- a/compositor/Planner.h
+++ b/compositor/Planner.h
@@ -36,7 +36,7 @@
   // Removes and returns the next available plane from planes
   static DrmPlane *PopPlane(std::vector<DrmPlane *> *planes) {
     if (planes->empty())
-      return NULL;
+      return nullptr;
     DrmPlane *plane = planes->front();
     planes->erase(planes->begin());
     return plane;