[media] soc_camera: Use soc_camera_device::sizeimage to compute buffer sizes

Instead of computing the buffer size manually in the videobuf queue
setup and buffer prepare callbacks, use the previously negotiated
soc_camera_device::sizeimage value.

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>