drm/exynos: create exynos_check_plane()

Split update plane in two parts, an initial check part that can fail
and the update part that can't fail.

This is a important step for the upcoming atomic modesetting support.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>