fix to nightly failure

Use num_mod_workers[MOD_CDEF_SEARCH] for
cdef search mt check.

Failure in SmallWidthMultiThreaded was triggered via:
https://aomedia-review.googlesource.com/c/aom/+/216501

Bug: 558463888, 559075253, 559225640
Change-Id: I6f9ff430ce58a80674ae9883fa9a6431668245dc

av1/encoder/pickcdef.c

diff --git a/av1/encoder/pickcdef.c b/av1/encoder/pickcdef.c
index 8ece9331..ed6db25 100644
--- a/av1/encoder/pickcdef.c
+++ b/av1/encoder/pickcdef.c
@@ -899,7 +899,7 @@
   // Allocate CDEF search context buffers.
   cdef_alloc_data(cm, cdef_search_ctx);
   // Frame level mse calculation.
-  if (cpi->mt_info.num_workers > 1) {
+  if (cpi->mt_info.num_mod_workers[MOD_CDEF_SEARCH] > 1) {
     av1_cdef_mse_calc_frame_mt(cpi);
   } else {
     cdef_mse_calc_frame(cdef_search_ctx, cm->error, adaptive_cdef_mode);