drm/exynos: atomic phase 3: atomic updates of planes

Now that phase 1 and 2 are complete we can switch the update/disable_plane
callbacks to their atomic version.

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>
Signed-off-by: Inki Dae <inki.dae@samsung.com>