msm: camera: isp: Define new callback for early PCR

Add reg_update callback to handle irq in the event that early
PCR is enabled. This callback will be invoked only for the
first frame after streamon.

Change-Id: I007e5d6380c624910685d4f1ae5669b3458d0b1f
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>