drm/exynos/mixer: remove mixer_resources sub-structure

mixer_resources adds only unnecessary redirection, removing it makes the
code shorter and cleaner.

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>