media: v4l2-mem2mem: use vb2_is_streaming()

Don't touch q->streaming directly, use the vb2_is_streaming()
function instead.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>