msm: camera: reqmgr: Fix frame sync issue on SAT mode

Sync link doesn't have the corresponding req in
req table if the sync link is streamed on after
corresponding req. Add a initial sync req variable
to record the startup request number.

Change-Id: I4b6c97f106a95b766ae94956e4a35357de752ca0
Signed-off-by: Depeng Shao <depengs@codeaurora.org>
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>