missing check store state when get tiflash_mpp info

[guo-shaoge]

https://github.com/tikv/client-go/blob/681fb6e537b5b114f07cbc9eb0d6564742a470aa/internal/locate/region_cache.go#L1832

Should also check store.state, otherwise may get tomestone or offline store. (s.GetState() == metapb.StoreState_Up)

[guo-shaoge]

/assign @guo-shaoge