msm: camera: memmgr: fix the concurrency issue while accessing the list

In corner cases, There is possibility of concurrent access of mapping
info list during alloc and map the buffer.

To protect that, added mutex lock before accessing the list.

CRs-Fixed: 2751906
Change-Id: I6d390a0650095c1056d7e538d28c8c7d530f50d4
Signed-off-by: Chandan Kumar Jha <cjha@codeaurora.org>