drm/exynos: use private plane for crtc

The crtc can use private plane instead it has overlay struct. It will be
helpful use plane feature from crtc later.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>