power: qcom: msm-core: Add mutex lock for ioctl

There can be use after free with multiple ioctl calls.
Add mutex lock when updating userspace power.

Change-Id: Ieae08d05478a462b19cf7f91b64267177eaebe84
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>