soc: qcom: msm_perf: add null policy checks for cpufreq policy

null policy is expected especially when kernel is booted with
maxcpus=1 argument.

Change-Id: I30b17c224db340f74abcfcc27f2c2c91d74fd99e
Signed-off-by: Kishore Sri venkata Ganesh Bolisetty <bsrivenk@codeaurora.org>