drm/exynos: add prepare and cleanup phases for planes

From: Gustavo Padovan <gustavo.padovan@collabora.co.uk>

.prepare_plane() and .cleanup_plane() allows to perform extra operations
before and after the update of planes. For FIMD for example this will
be used to enable disable the shadow protection bit.

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