msm: npu: Use worker thread to load NPU firmware

Load NPU firmware will be done during system boot up time and
it takes hundreds ms. To avoid increase system boot up time,
use worker thread to load it.

Change-Id: I93401891d2922d17755fe13328f746680a90fec7
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>