UPSTREAM: cpufreq: schedutil: irq-work and mutex are only used in slow path

Execute the irq-work specific initialization/exit code only when the
fast path isn't available.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 21ef57297b15a49b0c4dd4e7135c1a08e9a29a1c)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Change-Id: Icfd68f455ef71846d799fcd2d8ec6aa1bf59573e