Remove magic number usage in SurfaceFlinger

Test: Manual
Change-Id: I9d4c7e4ec8546314864e0625a832e9a0bb49e95b
diff --git a/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp b/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp
index 2b603cc..5dcd6e3 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp
@@ -2466,7 +2466,7 @@
         }
     }
 
-    if (mHwc1DisplayMap.count(0) == 0) {
+    if (mHwc1DisplayMap.count(HWC_DISPLAY_PRIMARY) == 0) {
         ALOGE("prepareAllDisplays: Unable to find primary HWC1 display");
         return false;
     }