msm: camera: isp: Init spinlock to avoid unexpected behaviour

Using spin lock without init may result in unexpected behaviour,
This change initialize ctx_lock during init.

CRs-Fixed: 2724911
Change-Id: I250e6b1359bf9f3e2f3ec1ee27f6df84a50b207d
Signed-off-by: Vikram Sharma <vikramsa@codeaurora.org>