msm: camera: reqmgr: init completion correctly

Init completion after enqueue task may cause completion uses lock
without initialization(spin lock magic issue). Move init completion
before enqueue to prevent it.

Change-Id: Ib882f4c603d04a6762c04e044dbcdee712beea35
Signed-off-by: Junzhe Zou <jnzhezou@codeaurora.org>