drm_hwcomposer: compile platform-specifc code as needed

Test: builds for ryu
Change-Id: If96d28919a893c5b0656af9ee602271f56293bac
diff --git a/Android.mk b/Android.mk
index f46276c..c5ccd36 100644
--- a/Android.mk
+++ b/Android.mk
@@ -71,8 +71,6 @@
 	glworker.cpp \
 	hwcutils.cpp \
 	platform.cpp \
-	platformdrmgeneric.cpp \
-	platformnv.cpp \
 	separate_rects.cpp \
 	virtualcompositorworker.cpp \
 	vsyncworker.cpp
@@ -83,8 +81,10 @@
 
 ifeq ($(strip $(BOARD_DRM_HWCOMPOSER_BUFFER_IMPORTER)),nvidia-gralloc)
 LOCAL_CPPFLAGS += -DUSE_NVIDIA_IMPORTER
+LOCAL_SRC_FILES += platformnv.cpp
 else
 LOCAL_CPPFLAGS += -DUSE_DRM_GENERIC_IMPORTER
+LOCAL_SRC_FILES += platformdrmgeneric.cpp
 endif
 
 LOCAL_MODULE := hwcomposer.drm