drm/exynos: fixed duplicated mode setting.

this patch fixes that when drm_crtc_helper_set_mode() is called,
mode data for hardware overlay and conntroller are updated two times.
for example, in case that drm_crtc_helper_set_mode() is called,
overlay_ops->commit() and manager_ops->commit() callbacks can be called
two times, first at drm_crtc_helper_set_mode() and second
at drm_helper_connector_dpms().

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>