sync: Permit out-of order sync users

A sync user like oneshot may wish to signal points on a timeline
out of order. In this case, the sync points cannot be collapsed
while merging. Don't collapse these points so they can be signaled
as intended by the user.

Change-Id: Ia191dc7812fd42a31d39dae17b0b14923fe67e78
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>
Signed-off-by: Sunil Khatri <sunilkh@codeaurora.org>