[media] s5p-fimc: Do not allow changing format after REQBUFS

Protecting the color format with vb2_is_streaming() is not sufficient
as this prevents changing the format only after VIDIOC_STREAMON.
To prevent the color format reconfiguration as soon as buffers
are allocated use vb2_is_busy() instead.
Also make the videobuf queue ops structure static.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>