Scaling is currently supported only for RGB framebuffers

Coefficients and algorithm which coefficients to select are taken
from BSP driver.
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201060550.10392-22-jernej.skrabec@siol.net