PM: runtime: Introduce pm_runtime_blocked()

Introduce a new helper function called pm_runtime_blocked()
for checking the power.last_status value indicating whether or not
enabling runtime PM for the given device has been blocked (which
happens in the "prepare" phase of system-wide suspend if runtime
PM is disabled for the given device at that point).

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/4632087.LvFx2qVVIh@rjwysocki.net