[media] omap3isp: Prepare/unprepare clocks before/after enable/disable

Clock enable (disable) is split in two operations, prepare and enable
(disable and unprepare). Perform both when enabling/disabling the ISP
clocks.

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>