drm/sun4i: Fix DE2 YVU handling

Function sun8i_vi_layer_get_csc_mode() is supposed to return CSC mode
but due to inproper return type (bool instead of u32) it returns just 0
or 1. Colors are wrong for YVU formats because of that.

Fixes: daab3d0e

 ("drm/sun4i: de2: csc_mode in de2 format struct is mostly redundant")
Reported-by: Roman Stratiienko <r.stratiienko@gmail.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Tested-by: Roman Stratiienko <r.stratiienko@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200901220305.6809-1-jernej.skrabec@siol.net