REPL: display should only ever be called on the repl backend task

During a bracketed paste the display call can be called from another thread, which is a problem for some packages.

The last 2 here are pasted in.

```
$ julia -t8,0
julia> struct Foo end

julia> Base.display(::Foo) = @show Threads.threadid()

julia> Foo()
Threads.threadid() = 1

julia> Foo()
Threads.threadid() = 1

julia> Foo()
Threads.threadid() = 2

julia> Foo()
Threads.threadid() = 2
```