drm/exynos: remove exported functions from exynos_drm_plane

Now that no one is using the functions exported by exynos_drm_plane due
to the atomic conversion we can make remove some of the them or make them
static.

v2: remove unused exynos_drm_crtc

v3: fix checkpatch error (reported by Joonyoung)

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Tested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>