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: I72939b73bc68709f483194a0fca9e7b5a12b2ce4
Signed-off-by: Da Hoon Pyun <dpyun@codeaurora.org>