Enable motion field estimation in DRL

Enable the use of motion field estimation in the dynamic motion
vector referencing system. With default experiments on, it improves
the compression performance:

lowres 1.2%
midres 1.5%

Change-Id: Ifc5b15a7239b5c3212ea50f326ab99d372034658
diff --git a/av1/common/alloccommon.c b/av1/common/alloccommon.c
index 69b2197..0db9ff6 100644
--- a/av1/common/alloccommon.c
+++ b/av1/common/alloccommon.c
@@ -85,6 +85,10 @@
     }
     aom_free(pool->frame_bufs[i].mvs);
     pool->frame_bufs[i].mvs = NULL;
+#if CONFIG_MFMV
+    aom_free(pool->frame_bufs[i].tpl_mvs);
+    pool->frame_bufs[i].tpl_mvs = NULL;
+#endif
     aom_free_frame_buffer(&pool->frame_bufs[i].buf);
 #if CONFIG_HASH_ME
     av1_hash_table_destroy(&pool->frame_bufs[i].hash_table);