drm/exynos: cleanup exynos_drm_plane

Remove the unused fields of struct exynos_drm_plane.

v2: Remove index_color as well, also unused (thanks Joonyoung).

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>