cpufreq: interactive: BUG_ON when tunables are NULL after init

When tunables are not available for events other than
CPUFREQ_GOV_POLICY_INIT in cpufreq_governor_interactive(), trigger a
panic instead of throwing a warning.

When the original warning happens, some race condition must have
occurred, and governor will be in a bad state even if it might still
run for a while. Panic directly so that it's easier to catch the
first race event.

Change-Id: I2dc1185cabfe72a63739452731fe242924d2cf45
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>