drm/exynos/mixer: move mode commit to enable callback

Mode commit should not be called for every plane separately. It is enough
to call it once in enable callback. The change also requires that
the interlace check is moved to mixer_commit. It should be done in
the same patch to avoid regression.

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>