cpufreq: schedutil: Cache tunables on governor exit

Currently when all the related CPUs from a policy go offline or the
governor is switched, cpufreq framework calls sugov_exit() that
frees the governor tunables. When any of the related CPUs comes back
online or governor is switched back to schedutil sugov_init() gets
called which allocates a fresh set of tunables that are set to
default values. This can cause the userspace settings to those
tunables to be lost across governor switches or when an entire
cluster is hotplugged out.
To prevent this, save the tunable values on governor exit. Restore
these values to the newly allocated tunables on governor init.

Change-Id: I671d4d0e1a4e63e948bfddb0005367df33c0c249
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>