msm: camera: cpp: Update output buffer index of userspace frame pointer

user space frame pointer does not have info on output buffer index
when sending frame for processing. After get_buf on vb2 buffer,
update buff index to user pointer. This is required especially for
partial frame payloads where get_buff is called only for first
payload and the same output buff index should be set for
remaining partial payloads.

Change-Id: I1ffe09bc87be2209e49e6010fd55df3dd3332402
Signed-off-by: Krishna Srinivas Kundurthi <krisri@codeaurora.org>