drm/sun4i: Improve VI scaling for DE2/DE3

VI planes support coarse scaling which helps to overcome VI scaler
limitations. While exact working of coarse scaling isn't known, it seems
that it just skips programmed amount of rows and columns. This is
especially useful for downscaling very big planes (4K down to 1080p).

Horizontal coarse scaling is currently used to fit one line to VI scaler
buffer.

Vertical coarse scaling is used to assure that VI scaler is actually
capable of processing framebuffer in one frame time.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190228200329.11128-4-jernej.skrabec@siol.net