Don't invalid region when first round cause by deadlineErr when using config timeout

[crazycs520]

Issue

1. Minimal reproduce step

In the following case, suppose the region leader is in store1.

	ca = replicaSelectorAccessPathCase{
		reqType:   tikvrpc.CmdGet,
		readType:  kv.ReplicaReadMixed,
		staleRead: false,
		timeout:   time.Second,
		label:     nil,
		accessErr: []RegionErrorType{ServerIsBusyErr, ServerIsBusyErr, DeadLineExceededErr},

2. What did you expect to see?

		expect: &accessPathResult{
			accessPath: []string{
				"{addr: store1, replica-read: true, stale-read: false}",
				"{addr: store2, replica-read: true, stale-read: false}",
				"{addr: store3, replica-read: true, stale-read: false}"},
			respErr:         "",
			respRegionError: fakeEpochNotMatch,
			backoffCnt:      0,
			backoffDetail:   []string{},
			regionIsValid:   true,
		},

3. What did you see instead?

		expect: &accessPathResult{
			accessPath: []string{
				"{addr: store1, replica-read: true, stale-read: false}",
				"{addr: store2, replica-read: true, stale-read: false}",
				"{addr: store3, replica-read: true, stale-read: false}"},
			respErr:         "",
			respRegionError: fakeEpochNotMatch,
			backoffCnt:      0,
			backoffDetail:   []string{},
			regionIsValid:   false,    // should be true ?
		},

When user sets tidb_kv_read_timeout and meet DeadLineExceededErr at first round, we needn't invalidate the region cache.