msm: camera: isp: removed mutext lock from irq context

get sof timestamp called from tasklet which intern
should not take mutex as it may sleep. This change
removes the mutext lock from irq context.

CRs-Fixed: 2794250
Change-Id: If73acde1a4b931e4bfc3650638f7f977e18cb3ea
Signed-off-by: Vikram Sharma <vikramsa@codeaurora.org>