[media] s5p-mfc: use vb2_is_streaming() to check vb2 queue status

The streaming field in struct vb2_queue is meant to be private and should
not be used by drivers directly, instead the vb2_is_streaming() function
should be used to check the videobuf2 queue streaming status.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>