Add HWC_DISPLAY_VIRTUAL and adjust display count constants

Both surfaceflinger and HWC implementations have been defining these
themselves, making future changes more difficult. Better to define
them centrally, where they can be updated easily.

Change-Id: I793158b858c84b2470b05f9ea77cc5a0c7e247f1
diff --git a/include/hardware/hwcomposer_defs.h b/include/hardware/hwcomposer_defs.h
index ce4723c..c69a4bc 100644
--- a/include/hardware/hwcomposer_defs.h
+++ b/include/hardware/hwcomposer_defs.h
@@ -182,12 +182,16 @@
 enum {
     HWC_DISPLAY_PRIMARY     = 0,
     HWC_DISPLAY_EXTERNAL    = 1,    // HDMI, DP, etc.
-    HWC_NUM_DISPLAY_TYPES
+    HWC_DISPLAY_VIRTUAL     = 2,
+
+    HWC_NUM_PHYSICAL_DISPLAY_TYPES = 2,
+    HWC_NUM_DISPLAY_TYPES          = 3,
 };
 
 enum {
     HWC_DISPLAY_PRIMARY_BIT     = 1 << HWC_DISPLAY_PRIMARY,
     HWC_DISPLAY_EXTERNAL_BIT    = 1 << HWC_DISPLAY_EXTERNAL,
+    HWC_DISPLAY_VIRTUAL_BIT     = 1 << HWC_DISPLAY_VIRTUAL,
 };
 
 /*****************************************************************************/