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.