[CPUFREQ] use deferrable delayed work init in conservative governor

Venki Pallipadi made a similar change to the ondemand governor a while
back (in commit 28287033e12463c8ff89f1ea8038783d0360391c). It seems to
work just as well in the conservative governor, leading to fewer wakeups
as reported by powertop.

Signed-off-by: Ben Slusky <sluskyb@paranoiacs.org>
Signed-off-by: Dave Jones <davej@redhat.com>