drm/exynos: remove exynos_plane_commit() wrapper

It's doing nothing but calling exynos_crtc->ops->win_commit(), so let's
call this directly to avoid extra layers of abstraction.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>