vp9: set kf_max_dist to a reasonable default (128)

the same as vp8, with the same reasoning from:
2a0d7b1 Reduce the default kf_max_dist to 128.

see also:
https://trac.ffmpeg.org/ticket/4904
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=815673

+ restore vpxenc behavior of taking the library default rather than
  forcing 5s

This change also exposes an issue with one-pass svc in cbr mode, keep
the old default in datarate_test.cc for now.

Change-Id: Id6d1244f42490b06fefc1a7b4e12a423a1f83e88

test/datarate_test.cc

@@ -905,6 +905,9 @@ TEST_P(DatarateOnePassCbrSvc, OnePassCbrSvc) {
   svc_params_.scaling_factor_num[1] = 288;
   svc_params_.scaling_factor_den[1] = 288;
   cfg_.rc_dropframe_thresh = 10;
+  // TODO(marpan): another test should be added for default/small kf_max_dist
+  // once https://bugs.chromium.org/p/webm/issues/detail?id=1150 is fixed.
+  cfg_.kf_max_dist = 9999;
   ::libvpx_test::I420VideoSource video("hantro_collage_w352h288.yuv", 352, 288,
                                        30, 1, 0, 200);
   // TODO(wonkap/marpan): Check that effective_datarate for each layer hits the