distinct datetime,timestamp return an unexpected result

## Bug Report

### 1. Minimal reproduce step

```
mysql> create table t(start_time datetime default '1970-01-01 10:00:00',end_time timestamp default '1970-01-01 10:00:00');
Query OK, 0 rows affected (0.08 sec)

mysql>
mysql> insert into t value();
Query OK, 1 row affected (0.01 sec)

mysql> select distinct start_time,end_time from t;
ERROR 1292 (22007): Incorrect datetime value: '1970-01-01 02:00:00'

mysql> select distinct start_time from t;
+---------------------+
| start_time |
+---------------------+
| 1970-01-01 10:00:00 |
+---------------------+
1 row in set (0.01 sec)

mysql> select distinct end_time from t;
+---------------------+
| end_time |
+---------------------+
| 1970-01-01 10:00:00 |
+---------------------+
1 row in set (0.00 sec)
```

### 2. What did you expect to see?
no error
### 3. What did you see instead
```
mysql> select distinct start_time,end_time from t;
ERROR 1292 (22007): Incorrect datetime value: '1970-01-01 02:00:00'
```
### 4. What is your TiDB version?
v6
v7
v8
v9