drm/exynos: atomic phase 3: use atomic .set_config helper

drm/exynos: atomic phase 3: use atomic .set_config helper

Now that phase 1 and 2 are complete switch .set_config helper to
use the atomic one.

v2: also remove .prepare() callback

v3: remove .mode_set() and .mode_set_base() and encoder's
.prepare() callbacks

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>