make @@tidb_distsql_scan_concurrency hint work for keep order request

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

I want @@tidb_distsql_scan_concurrency sql variable work:

select /*+ set_var(tidb_distsql_scan_concurrency=14)*/ * from t order by id;

2. What did you expect to see? (Required)

distsql scan concurrency is 14

3. What did you see instead (Required)

For order by request, the concurrency is lower to 2

4. What is your TiDB version? (Required)

master