media: atmel: atmel-isc-base: move cropping settings outside start_dma

The cropping configuration of the Parallel Front Engine (PFE) can be done
just once when the streaming starts. The ISC configuration is in place and
will not be changed while streaming.
It is not effective to keep rewriting the crop registers configuration
everytime start_dma is called, as this can be called for each queued
buffer.
Thus we can configure the cropping at start_streaming time.
This change moves the code to a dedicated function.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>