[media] omap3isp: Use monotonic timestamps for statistics buffers

V4L2 buffers use the monotonic clock, while statistics buffers use wall
time. This makes it difficult to correlate video frames and statistics.
Switch statistics buffers to the monotonic clock to fix this.

Reported-by: Antoine Reversat <a.reversat@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>