diag: dci: Add protection while querying event status

Currently, protection is missing when querying event
status due to which already removed dci client entry
might be accessed. This patch takes care of issue by
taking proper locking.

CRs-Fixed: 2015892
Change-Id: I4195c4c6198d85e96559f1728d74419527a76bc5
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>