drm/exynos: atomic check only enabled crtc states

Since atomic check is called also for disabled crtcs it should skip
mode checking as it can be uninitialized. The patch fixes it.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>