cgtop: Fix processing of controllers other than CPU

cgtop: Fix processing of controllers other than CPU

After debugging the issue with gdb, I found that the following change

 94ddb08 "cgtop: Still try to get CPU statistics if controller-free"

has introduced a bug, which prevents process(..) method processing
memory and io controllers when cpu_accounting_is_cheap() is true.
The obvious fix is to move this branch to be the last one, keeping
the intended behavior of the above change, without having a negative
effect on the other controllers.

Fixes #11773 [systemd-cgtop no longer shows memory (and io) usage]