drm/exynos: move exynos_drm_crtc_disable()

This is a preparation commit to move exynos_drm_crtc_disable() together
with the future exynos_drm_crtc_enable() that will come from the split of
exynos_drm_crtc_dpms() callback.

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