cpu_ref: fix c++11 narrowing violations

the best fix for these is to use static casting

Change-Id: I00344826a33e5387638ae5edf4c6b85c92f43769
diff --git a/cpu_ref/rsCpuIntrinsicYuvToRGB.cpp b/cpu_ref/rsCpuIntrinsicYuvToRGB.cpp
index 4e38459..22f0962 100644
--- a/cpu_ref/rsCpuIntrinsicYuvToRGB.cpp
+++ b/cpu_ref/rsCpuIntrinsicYuvToRGB.cpp
@@ -92,7 +92,8 @@
         p.z = 255;
     }
 
-    return (uchar4){p.x, p.y, p.z, p.w};
+    return (uchar4){static_cast<uchar>(p.x), static_cast<uchar>(p.y),
+                    static_cast<uchar>(p.z), static_cast<uchar>(p.w)};
 }