drm/exynos: remove leftover functions declarations

These functions were already removed by previous cleanup work, but these
ones were left behind.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>