msm: camera: sync: resolve race condition between merge and signal

During merge, it ensures the state of merged fence and remaining
count are serialized with any of the child node sync call.

Change-Id: I443a82a39c2b6a52b70fe66b131fc9972b363c85
Signed-off-by: Alok Pandey <akumarpa@codeaurora.org>
Signed-off-by: Junzhe Zou <jnzhezou@codeaurora.org>