msm: camera: core: Fix context release timing issue

When sync object is invalid and num_in_map > 1,
it will lead to context ref leak. Check sync
object first, then register sync call back.

Change-Id: I3418083c9f32ae0d7a9797d3e84dfda0a21385ee
Signed-off-by: Ayush Kumar <ayushkr@codeaurora.org>