drm/exynos: add exynos specific .atomic_commit()

exynos needs to update planes with the crtc enabled (mainly for the FIMD
case) so this specific atomic commit changes the order of
drm_atomic_helper_commit_modeset_enables() and
drm_atomic_helper_commit_planes() to commit planes after we enable crtc
and encoders.

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