[media] s5p-mfc: fix encoder crash after VIDIOC_STREAMOFF

VIDIOC_STREAMOFF clears the encoder's destination queue -- routines run
from the interrupt handler cannot assume that the queue is non-empty.

Signed-off-by: John Sheu <sheu@google.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>