drm/exynos: unify plane update on exynos_update_plane()

We can safely use the exynos_update_plane() to update the plane
framebuffer for both the overlay and primary planes.

Note that this patch removes a call to manager->ops->commit() in
exynos_drm_crtc_mode_set_commit(). The commit() call is used only by the
fimd driver to set underlying timings and need only in full modeset
operations. For plane update only win_commit is needed.

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