planner: error when access enum column by using a cached plan

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
create table tidx(e enum("a", "b", "c"), key(e));
prepare st from 'select * from t where e>?';
set @a=4;
execute st using @a;
```

### 2. What did you expect to see? (Required)
No error.

### 3. What did you see instead (Required)
```
ERROR 1265 (01000): Data truncated for column '%s' at row %d
```

### 4. What is your TiDB version? (Required)

```
| 5.7.25-TiDB-v6.7.0-alpha-168-gad01574a71 |
```