media: imx: capture: Move default format init to a separate function

To prepare for more complex default format initialization and keep
imx_media_capture_device_register() simple, extract the format
initialization code to a separate function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>