drm/exynos/mixer: move resolution configuration to single function

Screen resolution configuration depends on HW version, let's put it into
single function to make it consistent and simplify the code.

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>