drm/exynos: don't track enabled state at exynos_crtc

struct drm_crtc already stores the enabled state of the crtc
thus we don't need to replicate enabled in exynos_drm_crtc.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>