Reset mmx register in av1_simple_motion_search

BUG=aomedia:2467

Change-Id: I95e913a39bcdc82c193e1b19e40b686816df4d9d
diff --git a/av1/encoder/partition_strategy.c b/av1/encoder/partition_strategy.c
index 32ca1e7..4dcfe0c 100644
--- a/av1/encoder/partition_strategy.c
+++ b/av1/encoder/partition_strategy.c
@@ -497,6 +497,7 @@
                                          int *partition_horz_allowed,
                                          int *partition_vert_allowed,
                                          int *prune_horz, int *prune_vert) {
+  aom_clear_system_state();
   const AV1_COMMON *const cm = &cpi->common;
   const int bsize_idx = convert_bsize_to_idx(bsize);
   const int is_720p_or_larger = AOMMIN(cm->width, cm->height) >= 720;