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: I79ad09ae9d7ce8c627d72207085cfe58fb6649dc
Signed-off-by: Kui Wang <kuiw@codeaurora.org>