Force a repaint when hwc invalidate hook is called

without this prepare() would be called but not set() since the
dirty region is empty.

Change-Id: I038acfbdad4c16015357ccde4d1949391d6f989d
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.cpp b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
index 879e858..be9b226 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
@@ -75,7 +75,7 @@
 }
 
 void HWComposer::invalidate() {
-    mFlinger->signalEvent();
+    mFlinger->repaintEverything();
 }
 
 void HWComposer::setFrameBuffer(EGLDisplay dpy, EGLSurface sur) {