msm: camera: reqmgr: Change subdev register logic in camera req manager

Due to deferred probes being dpendent on request manager to register
v4l2 subdev, failures are seen if probes are delayed. Change logic
for subdev registration to avoid failures due to delay.

CRs-Fixed: 2584631
Change-Id: Ia01d5d27ff6a6e6dbbb8e3b25b9aa53ac2fa9e38
Signed-off-by: Mukund Madhusudan Atre <matre@codeaurora.org>
Signed-off-by: Jigarkumar Zala <jzala@codeaurora.org>