drm/sun4i: frontend: Apply format sub-sampling to CH1 dimensions

The frontend comes with two "channels", that can be configured
independently. When used in YUV mode, the first channel (CH0) represents
the luminance component while the second channel (CH1) represents the
chrominance. In RGB mode, both have to be configured the same way.

Use variables (with the YUV terminology) for each channel's
dimensions, calculating the chroma dimensions from the luma dimensions
and the sub-sampling factors from the format description.

Since the configured size only has pixel precision, the fractional
fixed-point part of the source size is dropped for both components to
ensure that the scaling factors are accurate.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181123092515.2511-26-paul.kocialkowski@bootlin.com