[NORMATIVE] mfmv only use the first valid mv in stack

This reduces the cycles spent on most add_tpl_ref_mv calls to
1/3 of what it was. In local profiling, the percentage on
av1_find_mv_refs + add_tpl_ref_mv goes down from 6.81% to 4.60%.

BUG=aomedia:1342

Change-Id: Id2f879766c4d0bbee41a35aff803757f760792d5
diff --git a/av1/common/mvref_common.c b/av1/common/mvref_common.c
index af25161..8896172 100644
--- a/av1/common/mvref_common.c
+++ b/av1/common/mvref_common.c
@@ -524,6 +524,7 @@
         }
 
         ++coll_blk_count;
+        return coll_blk_count;
       }
     }
   } else {
@@ -591,6 +592,7 @@
         }
 
         ++coll_blk_count;
+        return coll_blk_count;
       }
     }
   }