[media] s5p-jpeg: Ensure setting correct value of the chroma subsampling control

Exynos4x12 has limitations regarding setting chroma subsampling
of an output JPEG image. It cannot be lower than the subsampling
of the raw source image. Also in case of V4L2_JPEG_CHROMA_SUBSAMPLING_GRAY
option the source image fourcc has to be V4L2_PIX_FMT_GREY.
This patch implements try_ctrl callback containing mechanism that
prevents setting invalid value of the V4L2_CID_JPEG_CHROMA_SUBSAMPLING
control.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>