hwc: Handle video with plane alpha properly

An earlier patch disabled plane alpha completely for video.
Instead, do this only when we are in video only mode.

Change-Id: I2edc8c5ff5c90ec205b63175e5f3a4e6ff7eae69
diff --git a/libhwcomposer/hwc_mdpcomp.cpp b/libhwcomposer/hwc_mdpcomp.cpp
index e97d769..dbfb16b 100644
--- a/libhwcomposer/hwc_mdpcomp.cpp
+++ b/libhwcomposer/hwc_mdpcomp.cpp
@@ -569,6 +569,13 @@
         return false;
     }
 
+    if(layer->planeAlpha < 0xFF) {
+        ALOGD_IF(isDebug(), "%s: Cannot handle YUV layer with plane alpha\
+                 in video only mode",
+                 __FUNCTION__);
+        return false;
+    }
+
     return true;
 }