msm: npu: Avoid to access NPU registers when power is off

NPU SSR could be triggered when NPU power is off, so SSR handler
should avoid to access NPU registers before NPU power is turned on.

Change-Id: I0ea1fcf1e025dccc4d85ec0f6f0e7184115c3db0
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>