gralloc: Default flexible YUV format to NV21_ZSL

There could be buffers allocated in flexible YUV format without any
camera usage flags. Default the format for such buffers to NV21_ZSL.

Bug: 24197991
Change-Id: I25814aed652bfcddb19b411499bc8d64e76e081b

msm8226/libgralloc/gpu.cpp

@@ -286,6 +286,10 @@ int gpu_context_t::alloc_impl(int w, int h, int format, int usage,
         else if(usage & GRALLOC_USAGE_HW_COMPOSER)
             //XXX: If we still haven't set a format, default to RGBA8888
             grallocFormat = HAL_PIXEL_FORMAT_RGBA_8888;
+        else if(format == HAL_PIXEL_FORMAT_YCbCr_420_888)
+            // If no other usage flags are detected, default the flexible YUV
+            // format to NV21_ZSL.
+            grallocFormat = HAL_PIXEL_FORMAT_NV21_ZSL;
     }
 
     getGrallocInformationFromFormat(grallocFormat, &bufferType);