drm/exynos: mixer: Fix color format setting

Fix color format decision based on height(pixels).

According to CEA-861-E:
"High Definition (HD) - A CE video format that, inclusively, has between
720 to 1080 active vertical lines (Vactive) lines per video frame."

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>