msm:camera: Support batch processing in generic buf mgr

Support batch user buffer handling in generic buf mgr
to provide clients with list of buffer indexes based
on the container index from vb2.

Change-Id: Ide306d422915a8035555271bb8d30c3bbee9c44c
Signed-off-by: Hariram Purushothaman <hariramp@codeaurora.org>