Fix handling of undefined TARGET_FORCE_HWC_FOR_VIRTUAL_DISPLAYS

related to b/8316155 - Implement HWComposer 1.3 w/ virtual display support
Change-Id: I2dc8d65a74174aa833cc491264f97f1f2df49307
diff --git a/services/surfaceflinger/Android.mk b/services/surfaceflinger/Android.mk
index 1a8f243..b2bc550 100644
--- a/services/surfaceflinger/Android.mk
+++ b/services/surfaceflinger/Android.mk
@@ -51,10 +51,8 @@
 	LOCAL_CFLAGS += -DTARGET_DISABLE_TRIPLE_BUFFERING
 endif
 
-ifneq ($(TARGET_FORCE_HWC_FOR_VIRTUAL_DISPLAYS),)
-    LOCAL_CFLAGS += -DFORCE_HWC_COPY_FOR_VIRTUAL_DISPLAYS=false
-else
-    LOCAL_CFLAGS += -DFORCE_HWC_COPY_FOR_VIRTUAL_DISPLAYS=true
+ifeq ($(TARGET_FORCE_HWC_FOR_VIRTUAL_DISPLAYS),true)
+    LOCAL_CFLAGS += -DFORCE_HWC_COPY_FOR_VIRTUAL_DISPLAYS
 endif
 
 ifneq ($(NUM_FRAMEBUFFER_SURFACE_BUFFERS),)