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: I44236a7ae991c0cbd50c36c17cd4cb8e11705a7f
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>