hwc2: Handle multiple displays concurrency

Allow and limit concurrent displays to below combinations:
- BuiltIn + BuiltIn + External
- BuiltIn + BuiltIn + Virtual
- BuiltIn + External + External
- BuiltIn + External
- BuiltIn + Virtual

CRs-Fixed: 2324149
Change-Id: I87471585f73c411fb93fbe4fcc51e4191d636dc8
diff --git a/libqdutils/display_config.h b/libqdutils/display_config.h
index 8073bb1..8373c90 100644
--- a/libqdutils/display_config.h
+++ b/libqdutils/display_config.h
@@ -54,6 +54,7 @@
 
     // Additional displays only for vendor client (e.g. pp) reference
     DISPLAY_BUILTIN_2 = 3,
+    DISPLAY_EXTERNAL_2 = 4,
 };
 
 // External Display states - used in setSecondaryDisplayStatus()