[media] mx2_camera: Use soc_mbus_image_size() instead of manual computation

Use the new soc_mbus_image_size() function to compute the image size.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>