msm: npu: Access NPU registers after power on

subsystem_restart_dev may return before subsystem restart
finishes, so it's possible that NPU has not been powered on
after it returns. This change is to wait until NPU is powered
on before accessing registers. And it also changes to enables
interrupts until FW is loaded to avoid watchdog interrupt to be
triggered during PIL loads FW.

Change-Id: I8c754776d28a83dbf00222787a22a6a4575250d2
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>