Driver-initiated DMA jobs are synchronized jobs, i.e. the driver polls on
fence object until the job is finished. There is no interrupt from the
device. Therefore, no need to add space for 2 * msg_prot packets to the
end of the CB. Only a single msg_prot is needed (to write the fence).
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>