drm_hwcomposer: add ATRACE_CALL.

The DrmDisplayCompositor::CommitFrame() has been moved to a separate
worker thread so it's helpful to add it to systrace.

Change-Id: I82031f56ddc0587266f9a213208134f045f66b9a
diff --git a/drmdisplaycompositor.cpp b/drmdisplaycompositor.cpp
index 3f41fca..44c751d 100644
--- a/drmdisplaycompositor.cpp
+++ b/drmdisplaycompositor.cpp
@@ -585,6 +585,8 @@
 }
 
 int DrmDisplayCompositor::CommitFrame(DrmDisplayComposition *display_comp) {
+  ATRACE_CALL();
+
   int ret = 0;
 
   std::vector<DrmHwcLayer> &layers = display_comp->layers();