drm/exynos: introduce exynos_drm_plane_state structure

This patch introduces exynos_drm_plane_state structure, which subclasses
drm_plane_state and holds precalculated data suitable for configuring
Exynos hardware.

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