Update min_tx_size in reset_tx_size

BUG=aomedia:1267

Change-Id: I3b2c3a694c9d304cbe672c45a154ff3b29d02e24

av1/encoder/encodeframe.c

diff --git a/av1/encoder/encodeframe.c b/av1/encoder/encodeframe.c
index d8e9267..d7b7750 100644
--- a/av1/encoder/encodeframe.c
+++ b/av1/encoder/encodeframe.c
@@ -394,6 +394,7 @@
       for (int idx = 0; idx < xd->n8_w; ++idx)
         mbmi->inter_tx_size[idy][idx] = mbmi->tx_size;
     }
+    mbmi->min_tx_size = mbmi->tx_size;
   }
 }