drm/exynos: atomic phase 1: use drm_plane_helper_update()
Rip out the check from exynos_update_plane() and create
exynos_check_plane() for the check phase enabling use to use
the atomic helpers to call our check and update phases when updating
planes.
Update all users of exynos_update_plane() accordingly to call
exynos_check_plane() before.