msm: camera: isp: Obtain timestamp as part of frame header

In use-cases that involve custom HW retrieve the timestamp
for shutter from frame header as opposed to csid registers.

CRs-Fixed: 2524308
Change-Id: I5de789cf939546affbfe6d537d8090982f39189d
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>