msm: npu: Use mutex to protect power level change

NPU power level can be changed by multiple sources, so it's
necessary to use mutex to protect it while it's updated by
one of the sources.

Change-Id: I3e99866f0f279fac16463055867225402c03e369
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>