drm/exynos: fix point to call overlay_ops->mode_set

Call overlay->mode_set from crtc->mode_set instead of encoder->mode_set,
it makes codes clearly.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>