cpufreq: interactive: Cache tunables when they are created

Currently, tunables are only saved to per_cpu field when
CPUFREQ_GOV_POLICY_EXIT event happens. Save tunables the moment they
are created so that per_cpu cached_tunables field always matches
the tunables in use. This is useful for modifying tunable values
across clusters.

Change-Id: I9e30d5e93d6fde1282b5450458d8a605d568a0f5
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>