[media] atmel-isi: move configure_geometry() to start_streaming()

As in set_fmt() function we only need to know which format is been set,
we don't need to access the ISI hardware in this moment.

So move the configure_geometry(), which access the ISI hardware, to
start_streaming() will make code more consistent and simpler.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>