drm/exynos: use drm atomic state directly

For some fields the use of struct exynos_drm_plane filled with data from
the plane state just creates a source of duplicated information and
overhead.  Here we change the crtc drivers to access the plane state
directly simplifying the code by not relying on a exynos internal struct.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>