[media] media: fsl-viu: fix support for streaming with mmap method

Streaming using mmap didn't work in the VIU driver. We need to
start/stop DMA in streamon/streamoff and free the buffers on
release. Add appropriate driver extension now.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>