drm/exynos: Constify exynos_drm_crtc_ops

The Exynos DRM code does not modify the ops provided by CRTC driver in
exynos_drm_crtc_create() call.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>