Move the initialization of mb_wiener_mt_exit flag

This CL moves the initialization of 'mb_wiener_mt_exit'
flag from row_mt_mem_alloc() to av1_calc_mb_wiener_var_mt(),
as row_mt_mem_alloc() is not invoked during the
multithreading of wiener variance computation.

Bug: aomedia:3276
Change-Id: I59cdca8b983390523bb0b0e100f8e00ec814e98d

av1/encoder/ethread.c

diff --git a/av1/encoder/ethread.c b/av1/encoder/ethread.c
index 5493fc4..24d47f3 100644
--- a/av1/encoder/ethread.c
+++ b/av1/encoder/ethread.c
@@ -274,7 +274,6 @@
   enc_row_mt->allocated_sb_rows = sb_rows;
   enc_row_mt->row_mt_exit = false;
   enc_row_mt->firstpass_mt_exit = false;
-  enc_row_mt->mb_wiener_mt_exit = false;
 }
 
 void av1_row_mt_mem_dealloc(AV1_COMP *cpi) {
@@ -2836,6 +2835,7 @@
   intra_row_mt_sync->next_mi_row = 0;
   memset(intra_row_mt_sync->num_finished_cols, -1,
          sizeof(*intra_row_mt_sync->num_finished_cols) * mi_rows);
+  mt_info->enc_row_mt.mb_wiener_mt_exit = false;
 
   prepare_wiener_var_workers(cpi, cal_mb_wiener_var_hook, num_workers);
   launch_workers(mt_info, num_workers);