maxwell3d: Add missing return in default SizeInBytes() case

We were returning '1' in ComponentCount()'s default case but were
neglecting to do the same with SizeInBytes().

src/video_core/engines/maxwell_3d.h

@@ -242,6 +242,7 @@ public:
                     return 4;
                 default:
                     UNREACHABLE();
+                    return 1;
                 }
             }