teach HWC about multiple displays

Change-Id: I5e72a83d419a729835cb0e1ec45557b4d3fb56b1
diff --git a/services/surfaceflinger/DisplayDevice.h b/services/surfaceflinger/DisplayDevice.h
index 14b194f..ea56d90 100644
--- a/services/surfaceflinger/DisplayDevice.h
+++ b/services/surfaceflinger/DisplayDevice.h
@@ -28,6 +28,8 @@
 #include <utils/Mutex.h>
 #include <utils/Timers.h>
 
+#include <hardware/hwcomposer_defs.h>
+
 #include "Transform.h"
 
 struct ANativeWindow;
@@ -50,9 +52,9 @@
     Region undefinedRegion;
 
     enum {
-        DISPLAY_ID_MAIN = 0,
-        DISPLAY_ID_HDMI = 1,
-        DISPLAY_ID_COUNT
+        DISPLAY_ID_MAIN = HWC_DISPLAY_PRIMARY,
+        DISPLAY_ID_HDMI = HWC_DISPLAY_EXTERNAL,
+        DISPLAY_ID_COUNT = HWC_NUM_DISPLAY_TYPES
     };
 
     enum {