drm/atmel-hlcdc: use helper to get crtc state

DRM core provide helper to access crtc state.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1458045988-25246-1-git-send-email-a.hajda@samsung.com