don't call hwc with non-empty layer lists for blanked displays

we now make sure to take the blanked state of a display into
account when we build its list of visible layers, this ensures
that we won't call prepare/set with a non-empty list when the
display is blanked.

Possibly fixes 7075380, 7103553, 7130187, 7016215

Bug: 7075380
Change-Id: I9fdd2e73d1b7621eaeca3d4ac2ae60d9fb1b3631
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index ea03e2d..ffe68c6 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -351,8 +351,8 @@
      * Compositing
      */
     void invalidateHwcGeometry();
-    void computeVisibleRegions(const LayerVector& currentLayers,
-            uint32_t layerStack,
+    static void computeVisibleRegions(
+            const LayerVector& currentLayers, uint32_t layerStack,
             Region& dirtyRegion, Region& opaqueRegion);
 
     void preComposition();