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.

CRs-Fixed: 2594185
Change-Id: I2d39ce3ea43bbe7bc05420b86b37fdfba4aa795a
Signed-off-by: Ayush Kumar <ayushkr@codeaurora.org>