rtc: Re-enable speed feature for speed 6.

Re-enable the speed feature used in rd_use_partition(),
for variance_partitioning realtime mode, that was disabled
as speculative fix to the issue below. The failure/crash
is still happening so this speed feature is not the issue.

The speed feature was disabled here:
https://aomedia-review.googlesource.com/c/aom/+/205381

Bug: b:457951958
Change-Id: Idb9f7bb78460c6d1bae54cdb3600543845bae2a2

av1/encoder/speed_features.c

diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index ef45021..e4c5e94 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -2831,8 +2831,11 @@
 
   if (cpi->oxcf.mode == REALTIME) {
     if (speed >= 6) {
-      // Disable this as speculative fix to issue: 457951958.
-      sf->part_sf.adjust_var_based_rd_partitioning = 0;
+      const int qindex_thresh = boosted ? 190 : (is_720p_or_larger ? 120 : 150);
+      sf->part_sf.adjust_var_based_rd_partitioning =
+          frame_is_intra_only(cm)
+              ? 0
+              : cm->quant_params.base_qindex > qindex_thresh;
     }
     return;
   }