We already have the plane pointer in before calling .update_plane() or
disable_plane() so pass it directly to those calls avoiding a new
conversion from zpos to struct exynos_drm_plane.

v2: don't remove check for suspended in FIMD (comment by Joonyoung)
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>