Use 8x8 block step size for spatial mv search

Use 8x8 block as minimum step size for reference motion vector
search.

Change-Id: Ie751dfa7bf7fe72e64843d5c1f490766e8b451de

av1/common/mvref_common.c

diff --git a/av1/common/mvref_common.c b/av1/common/mvref_common.c
index f85b97d..92a3df3 100644
--- a/av1/common/mvref_common.c
+++ b/av1/common/mvref_common.c
@@ -196,6 +196,7 @@
           xd->mi[mi_pos.row * xd->mi_stride + mi_pos.col];
       const MB_MODE_INFO *const candidate = &candidate_mi->mbmi;
       int len = AOMMIN(xd->n8_w, mi_size_wide[candidate->sb_type]);
+      if (abs(row_offset) > 1) len = AOMMAX(len, mi_size_wide[BLOCK_8X8]);
       if (use_step_16) len = AOMMAX(mi_size_wide[BLOCK_16X16], len);
       newmv_count += add_ref_mv_candidate(
           candidate_mi, candidate, rf, refmv_count, ref_mv_stack,
@@ -255,6 +256,7 @@
           xd->mi[mi_pos.row * xd->mi_stride + mi_pos.col];
       const MB_MODE_INFO *const candidate = &candidate_mi->mbmi;
       int len = AOMMIN(xd->n8_h, mi_size_high[candidate->sb_type]);
+      if (abs(col_offset) > 1) len = AOMMAX(len, mi_size_high[BLOCK_8X8]);
       if (use_step_16) len = AOMMAX(mi_size_high[BLOCK_16X16], len);
       newmv_count += add_ref_mv_candidate(
           candidate_mi, candidate, rf, refmv_count, ref_mv_stack,