msm: camera: isp: Fix boot ts when custom is enabled

When custom is enabled, we can generate boot time independent
of qtimer update from CSID.

CRs-Fixed: 2753196
Change-Id: I39255ddcc419d9a89981e57e8d3a9f466c62e7e6
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>