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: I5cd37e7e6fedb283f97af672175a9ae93a0a89a5
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>