msm: npu: Use mutex to protect map/unmap to avoid race condition

If multiple processes call map/unmap funciton concurrently, it's
possible to cause data structure corruption while both of them
accesses the same buffer at the same time. This change is to use
mutex to avoid this problem.

Change-Id: I3aeab502566c4105695a5d14736c9bea73d060e5
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>