drm/sun4i: frontend: Determine input format based on colorspace

Since all the RGB input formats have the same value for the DATA_FMT
field of the INPUT_FMT register, we can group them when the format is
known to be RGB. Here, we assume that a non-YUV format is RGB, because
the hardware does not support any other colorspace than RGB and YUV.

Use the DRM format info structure to check whether the format uses a
YUV colorspace.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190118145133.21281-6-paul.kocialkowski@bootlin.com