Remove the unused fields of struct exynos_drm_plane.

v2: Remove index_color as well, also unused (thanks Joonyoung).
Signed-off-by: NTobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: NGustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: NInki Dae <inki.dae@samsung.com>