drm/exynos: mixer: Fix color range setting

Color format and color range was set based on resolution. Change that,
by splitting range and format. Leave color format setting as it is,
set color range based on drm_display_mode using
drm_default_quant_range helper function.

Tested on Odroid-U3 with Exynos 4412 CPU, kernel next-20181128
using modetest.

Signed-off-by: Christoph Manszewski <c.manszewski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>