drm/exynos: call exynos_update_plane() directly on page flips

Avoid an extra call to exynos_drm_crtc_mode_set_commit() that only calls
exynos_update_plane().

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