Reduce fp mv search step on speed 5+

Performance:
 SPD_SET | TESTSET | AVG_PSNR  | OVR_PSNR  |   SSIM  |  SPD
---------+---------+-----------+-----------+---------+-------
         |  HDRES  |  +0.026%  |  +0.028%  | +0.110% | +1.0%
    6    | MIDRES  |  +0.005%  |  +0.002%  | +0.063% | +1.0%
         | LOWRES  |  -0.051%  |  -0.010%  | +0.028% | +0.6%
---------+---------+-----------+-----------+---------+-------
         |  HDRES  |  -0.080%  |  -0.082%  | -0.016% | +0.7%
    5    | MIDRES  |  +0.018%  |  +0.035%  | +0.024% | +0.8%
         | LOWRES  |  +0.018%  |  +0.029%  | +0.106% | +0.4%
---------+---------+-----------+-----------+---------+-------

STATS_CHANGED

Change-Id: Ic65bf18c7505ee296b13046c3f0376d066d5869d
diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index bd7ca67..3dd78ea 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -1033,6 +1033,8 @@
   }
 
   if (speed >= 5) {
+    sf->fp_sf.reduce_mv_step_param = 4;
+
     sf->part_sf.simple_motion_search_prune_agg = 3;
     sf->part_sf.ext_partition_eval_thresh =
         allow_screen_content_tools ? BLOCK_8X8 : BLOCK_16X16;