tidb_enforce_mpp doesn't work when keep order is true for table scan

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```
drop table if exists tt1;
create table tt1(c1 int, c2 int, primary key(c1));
insert into tt1 values(1, 1), (2, 2), (3, 3);
alter table tt1 set tiflash replica 1;
set @@tidb_enforce_mpp = 1;
set @@tidb_allow_tiflash_cop = true;
explain select * from tt1 where c1> 10 order by c1 limit 100;
```

### 2. What did you expect to see? (Required)
tiflash use mpp
### 3. What did you see instead (Required)
tiflash still use cop
### 4. What is your TiDB version? (Required)
```
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v8.3.0-alpha-4-g7e73ddc
Edition: Community
Git Commit Hash: 7e73ddc91b5f9f089e84f0dc645cf71a27b2ad50
Git Branch: HEAD
UTC Build Time: 2024-07-26 01:44:57
GoVersion: go1.21.10
Race Enabled: false
Check Table Before Drop: false
Store: tikv |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.001 sec)
```