debugd: add drm vblank events to systrace

Add drm::drm_vblank_event to gfx events.

BUG=chromium-os:17379 chromium:118032
TEST=collect trace and check contents

Change-Id: I36e83c145c7a53e071d88a8af20f48941a93929e
diff --git a/src/helpers/systrace.sh b/src/helpers/systrace.sh
index a794710..59ec97a 100755
--- a/src/helpers/systrace.sh
+++ b/src/helpers/systrace.sh
@@ -41,6 +41,7 @@
     i915:i915_flip_request
     i915:i915_flip_complete
     i915:i915_gem_object_pwrite
+    drm:drm_vblank_event
 "
 power_events="
     power:cpu_idle