mem-ruby: Cleanup replacement_data usage

mem-ruby: Cleanup replacement_data usage

The replacement_data can be assigned as soon as a block is allocated.
With this cleanup the lookup function can be used to avoid code
duplication.

Change-Id: I7561fddaa3ed348866699ecaf1e6aa477ba0bc9a
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23945
Reviewed-by: John Alsop <johnathan.alsop@amd.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>