msm: camera: sync: Add callback mechanism for signaled objects

Previously all objects that need to be signaled were added to a
list by dynamically allocating memory. This change dispatches
the necessary callbacks as and when it finds that an object is
signaled and ready to be dispatched.

Change-Id: I99759143c70dc7dd59ca72927c7e8b83521145ce
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>