hwc: Avoid MDP composition with alpha scaling

MDP4 supports scaling with alpha, only in RGB pipes.
In some cases this scaling happened with video pipes.
We now fall back to GLES composition for layers that have
alpha and need scaling.

Change-Id: I084355bad1cdb17c7308077b56006c59e75d2042
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Bug: 7648647
diff --git a/libhwcomposer/hwc_mdpcomp.cpp b/libhwcomposer/hwc_mdpcomp.cpp
index 4584207..8616719 100644
--- a/libhwcomposer/hwc_mdpcomp.cpp
+++ b/libhwcomposer/hwc_mdpcomp.cpp
@@ -273,6 +273,11 @@
         return false;
     }
 
+    if(ctx->listStats[HWC_DISPLAY_PRIMARY].needsAlphaScale) {
+        ALOGD_IF(isDebug(), "%s: frame needs alpha downscaling",__FUNCTION__);
+        return false;
+    }
+
     //FB composition on idle timeout
     if(sIdleFallBack) {
         sIdleFallBack = false;