media: coda: Add control for h.264 chroma qp index offset

Allow to set a fixed quantization parameter offset between luma and
chroma in the h.264 encoder.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>