msm: camera: Replace mutex lock with context spin lock

This change removes the context mutex lock for the entire
function when applying a request. With this change, the lock
window is reduced thereby improving performance.

Change-Id: Ibc387173b4da0d2c92604a01e9123572019b6469
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>