Add support for plane-alpha in HWC

Change-Id: I218e7dd5f23de535aabce61e993002ab6cb46cdd
diff --git a/services/surfaceflinger/LayerBase.cpp b/services/surfaceflinger/LayerBase.cpp
index f129607..54c51bb 100644
--- a/services/surfaceflinger/LayerBase.cpp
+++ b/services/surfaceflinger/LayerBase.cpp
@@ -283,7 +283,7 @@
         layer.setTransform(finalTransform);
     }
 
-    if (!isOpaque()) {
+    if (!isOpaque() || s.alpha != 0xFF) {
         layer.setBlending(mPremultipliedAlpha ?
                 HWC_BLENDING_PREMULT :
                 HWC_BLENDING_COVERAGE);