When --mpd is true, run _mpd variants first, then non-mpd.

This makes it easier to benchmark _mpd variants in a profiler.

E.g.,

<profiler> out/Release/nanobench --images --config 8888 --loops -1 --match sp_desk_nytimes

BUG=skia:

Review URL: https://codereview.chromium.org/1184673006

bench/nanobench.cpp

@@ -583,10 +583,10 @@ public:
             exit(1);
         }
 
-        fUseMPDs.push_back() = false;
         if (FLAGS_mpd) {
             fUseMPDs.push_back() = true;
         }
+        fUseMPDs.push_back() = false;
 
         // Prepare the images for decoding
         for (int i = 0; i < FLAGS_images.count(); i++) {