msm: camera: reqmgr: Hold device lock while retrieving session info

There is a possibility for destroy session and link to occur
concurrently, if this does we end up using a freed pointer while
linking. Avoid this by increasing the scope of the device lock.

Change-Id: I6ffb2d0633455a838cca9194e4717d82a9647183
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>