msm: Camera: Add buf entries only after successful mapping

If user buf mapping is wrong, then also those entries use to get
added in the queue head. That can cause failure in some corner cases.
Add buffer in the queue only after mapping is successful.

Change-Id: I1c104365b82bc4b9d34613aefff95a7e9f719432
Signed-off-by: Suman Mukherjee <sumam@codeaurora.org>