drm/exynos: remove unused prototype for crtc

exynos_drm_crtc_wait_pending_update, exynos_drm_crtc_finish_update
are not used anymore.

Signed-off-by: Kwanghoon Son <k.son@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>