PM / Domains: Extract code to power off/on a PM domain

PM domains are powered on/off from various places. Some callers do
latency measurements, others don't. Consolidate using two helper
functions, which always measure the latencies, and update the stored
latencies when needed.

Other minor changes:
  - Use pr_warn() instead of pr_warning(),
  - There's no need to check genpd->name, %s handles NULL pointers fine,
  - Make the warning format strings identical, to save memory.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>