State should not be sync when replica count is not enough

Please answer these questions before submitting your issue. Thanks!

1. Start 4 TiKV nodes with asynchronous replication mode, two of them are configured with `zone: z1`, the others are configured with `zone: z2`.
2. Wait for all region are balanced.
3. Configure label, primary, dr, primary-replicas, dr-replicas.
4. Enable dr-auto-sync.
5. Wait for PD says it's sync.
6. Kill all z2 nodes.

After a while, there are still some regions can't serve requests. They have two replicas placed in 2 z2 nodes.