drm_hwcomposer: Simplify default vsync period

VSyncWorker::SetVsyncPeriod won't be called with a value of 0, so the
only time the internal vsync period will be zero is if SetVsyncPeriod
was not called.

Set the 60Hz vsync period as a default value, and let the synthetic
vsync code expect it to be set to a reasonable value.

Change-Id: Ie1f83b0c11504acf304cc241923bfa60a5ffd647
Signed-off-by: Drew Davenport <ddavenport@google.com>
diff --git a/drm/VSyncWorker.h b/drm/VSyncWorker.h
index 2ee7f37..aff34d8 100644
--- a/drm/VSyncWorker.h
+++ b/drm/VSyncWorker.h
@@ -62,8 +62,10 @@
   bool thread_exit_ = false;
   int64_t last_timestamp_ = -1;
 
+  // Default to 60Hz refresh rate
+  static constexpr uint32_t kDefaultVSPeriodNs = 16666666;
   // Needs to be threadsafe.
-  uint32_t vsync_period_ns_ = 0;
+  uint32_t vsync_period_ns_ = kDefaultVSPeriodNs;
 
   std::condition_variable cv_;
   std::thread vswt_;