msm: camera_v2: protect cmd_ack_q around mutex

Insertions and deletions into the session based command
ack queue need to be synchronized since they may be accessed
from more than one context.

CRs-fixed: 516599
Change-Id: Ia8b4c10666ceede41352baa652f28e7e4ed8e5fc
Signed-off-by: Ankit Premrajka <ankitp@codeaurora.org>