msm: camera2: Add v4l2 subdev call to generic buffer manager.

The generic buffer manager needs to provide v4l2 subdevice call
interface for other kernel subdevices. Change the mutex to
spinlock so that other kenel subdevices can use.

Change-Id: I54b16328c5401710c0972dad7c8b9c9235f54906
Signed-off-by: Rajakumar Govindaram <rajakuma@codeaurora.org>