media: rockchip: rga: Only set output CSC mode for RGB input

[ Upstream commit 0f879bab72f47e8ba2421a984e7acfa763d3e84e ]

Setting the output CSC mode is required for a YUV output, but must not
be set when the input is also YUV. Doing this (as tested with a YUV420P
to YUV420P conversion) results in wrong colors.

Adapt the logic to only set the output CSC mode when the output is YUV and
the input is RGB. Also add a comment to clarify the rationale.

Fixes: f7e7b48e ("[media] rockchip/rga: v4l2 m2m support")
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>