drm/exynos: remove legacy ->suspend()/resume()

These legacy helpers should only be used by shadow-attaching drivers.
KMS drivers has its own way to handle suspend/resume and don't need to
use these two helpers.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <daeinki@gmail.com>