CEA 861 standard requestis that RGB quantization range is "limited" for
CEA modes. Support that by adding CSC matrix which downscales values.

This allows proper color reproduction on TV and PC monitor at the same
time. In future, override property can be added, like "Broadcast RGB"
in i915 driver.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304232512.51616-4-jernej.skrabec@siol.net