msm-core: Check for NULL pointer deference

In scenarios, where userspace hasn't populated the power numbers, per-cpu
pointer might be NULL. Add check to prevent any NULL pointer deferences.

Change-Id: Id193634dc764cb255a58026370ec7f9ef8abeebb
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>