msm: cpp: Handle batch buffers on preview identity

In batch mode with duplication use case, video buffer needs to be
primary buffer and preview buffer the duplicate buffer. If preview
buffer is the primary buffer batch video output is not generated
resulting in green frames. Swap the preview and video identity if
the batch buffers are sent on preview identity and duplication is
enabled.

CRs-Fixed: 952085
Change-Id: I46f63189e784ec388969a756b8e32c1180640478
Signed-off-by: Krishnankutty Kolathappilly <kkolatha@codeaurora.org>