PM / devfreq: Restart previous governor if new governor fails to start

If the new governor fails to start, switch back to old governor so that the
devfreq state is not left in some weird limbo.

Change-Id: I7cf1e6ceb63d27ce08b2d17b97a9844d257464ce
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>