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.

[Myungjoo: assume fatal on revert failure and set df->governor to NULL]
Signed-off-by: Sibi Sankar <sibis@codeaurora.org>
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>