Set motion_mode as SIMPLE_TRANSLATION in temporal_filtering

Otherwise, will trigger assertions in motion_mode validity check in
av1_build_inter_predictor called by av1_temporal_filter() because
motion_mode and inter/intra are all random values.

BUG=aomedia:1677

Change-Id: Ib120f8db4f3ce1e714c63fec903c190643889dc5
diff --git a/av1/encoder/temporal_filter.c b/av1/encoder/temporal_filter.c
index 84c92cb..3a41e90 100644
--- a/av1/encoder/temporal_filter.c
+++ b/av1/encoder/temporal_filter.c
@@ -360,6 +360,7 @@
 
         mbd->mi[0]->mv[0].as_mv.row = 0;
         mbd->mi[0]->mv[0].as_mv.col = 0;
+        mbd->mi[0]->motion_mode = SIMPLE_TRANSLATION;
 
         if (frame == alt_ref_index) {
           filter_weight = 2;