FROMLIST: psi: split update_stats into parts

Split update_stats into collect_percpu_times and update_averages for
collect_percpu_times to be reused later inside psi monitor.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>

(not upstream yet, latest version published at: https://lore.kernel.org/patchwork/patch/1052419/

)

Bug: 127712811
Bug: 129157727
Test: lmkd in PSI mode
Change-Id: Ic5dca1924a3f8997b49b5d16289f53bcc43b88fa
Signed-off-by: Suren Baghdasaryan <surenb@google.com>