drm/exynos: remove exynos_drm_crtc_plane_* wrappers

This functions were doing nothing but calling a manager op function,
so remove them and call the manager directly.

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