msm: npu: memset npu_kevent to prevent leak stack information

npu_kevent will be copied to user buffer, so it is possible that
user will be able to access some stack memory via the uninitialized
padding data. This change is to use memset to make sure this stack
information won't be leaked.

Change-Id: I77acafe11e67093f17906176720508674e1d35e8
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>