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.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Tested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de&gt;y>
Signed-off-by: Inki Dae <inki.dae@samsung.com>