[media] i.MX2: eMMa-PrP: Allow userptr IO mode

Userptr can be very useful if this device
is requested to use video buffers allocated
by another processing device. So that
buffers don't need to be copied.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>