drm/exynos: Fix potential NULL pointer dereference in exynos_drm_encoder.c

Check overlay_ops is not NULL as checked in the previous 'if' condition.
Fixes the following smatch error:
drivers/gpu/drm/exynos/exynos_drm_encoder.c:509 exynos_drm_encoder_plane_disable()
error: we previously assumed 'overlay_ops' could be null (see line 499)

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>