hwc: Drop alpha from writeback

The alpha in the writeback is unused right now. Force MDP to use
RGBX for composing writeback.
Bug: 11479817

Change-Id: I87e3fff962283af05a07d28698811a4402f2606c
diff --git a/msm8974/libhwcomposer/hwc.cpp b/msm8974/libhwcomposer/hwc.cpp
index b26c32a..87ec5c7 100644
--- a/msm8974/libhwcomposer/hwc.cpp
+++ b/msm8974/libhwcomposer/hwc.cpp
@@ -508,8 +508,11 @@
         ) {
 
             private_handle_t *ohnd = (private_handle_t *)list->outbuf;
+            int format = ohnd->format;
+            if (format == HAL_PIXEL_FORMAT_RGBA_8888)
+                format = HAL_PIXEL_FORMAT_RGBX_8888;
             Writeback::getInstance()->setOutputFormat(
-                    utils::getMdpFormat(ohnd->format));
+                    utils::getMdpFormat(format));
 
             int fd = -1; //FenceFD from the Copybit
             hwc_sync(ctx, list, dpy, fd);