msm: cpp: Query buffer from buffer manager based on index

Query buffer from buffer manager based on buf index.
This allows modules to provide a buffer associated
with a particular request and avoids a wrong buffer
from being returned.

CRs-Fixed: 1018651
Change-Id: I206f3fa334d96e9f57fcbd985922a436ed701ff3
Signed-off-by: Krishnankutty Kolathappilly <kkolatha@codeaurora.org>
Signed-off-by: Hariram Purushothaman <hariramp@codeaurora.org>