drm/exynos/mixer: enable support for 1024x768 and 1280x1024 modes

Since HDMI can handle these modes despite of MIXER limitations let's
enable them.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>