mem-ruby: Use lookup function in cache

mem-ruby: Use lookup function in cache

There is a function to perform lookups; there is no need to replicate
its code everywhere.

Change-Id: I1290594615d282722cd91071be8ef3c372414e4e
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23946
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>