drm/exynos: crtc: do not wait for the scanout completion

This patch removes exynos_drm_crtc_complete_scanout function call
which makes sure for overlay data to be updated to real hardware
when drm driver is released.

With atomic modeset support, it doesn't need the funtion anymore
because atomic modeset interface makes sure that.

Signed-off-by: Inki Dae <inki.dae@samsung.com>