[media] atmel-isc: start dma in some scenario

If a new vb buf is added to vb queue, the queue is
empty and steaming, dma should be started.

Signed-off-by: Songjun Wu <songjun.wu@microchip.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>