msm: camera: sync: Validate if fence is already signaled

This change provides a debugfs to switch between enqueuing
the cb for a signaled fence onto the workq or directly
invoking the cb in the same thread context. By default
the task will be enqueued onto the workq.

To have the sync driver invoke cb in same ctx:
adb shell "echo Y >
	/sys/kernel/debug/camera_sync/trigger_cb_without_switch"

Change-Id: Ic2fd7e0f3b08224020fcd2bd6f317e0da1939d10
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>