[media] omap3isp: Platform data could be NULL

Only check for call platform data callback functions if there's platform
data. Also take care of a few other cases where the NULL pdata pointer could
have been accessed, and remove the check for NULL dev->platform_data
pointer.

Removing the check for NULL dev->platform_data isn't strictly needed by the
DT support but there's no harm from that either: the device now can be used
without sensors, for instance.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>