hwc: Add support for HDMI vsync

SurfaceFlinger can enable HDMI vsync through event control.
Poll for HDMI vsync events and report them to SurfaceFlinger.

Change-Id: I301f1dffbab56b3b5a3dda5a59c37721688d7849
diff --git a/libexternal/external.h b/libexternal/external.h
index d35490f..7ab7c3f 100644
--- a/libexternal/external.h
+++ b/libexternal/external.h
@@ -58,6 +58,7 @@
     int  configureWFDDisplay();
     int  teardownHDMIDisplay();
     int  teardownWFDDisplay();
+    int getHDMIIndex() { return mHdmiFbNum; }
 
 private:
     void setSPDInfo(const char* node, const char* property);