drm/exynos: fimd: remove unnecessary waiting vblank routine

The exynos_drm_crtc_dpms() waits until pended page flip
queue is empty, calls the drm_vblank_off() then calls
manager->ops->dpms() when mode is DRM_MODE_DPMS_OFF.
The fimd_dpms() is one of manager->ops->dpms()s and
finally calls fimd_window_suspend().
But there is no active window and vblank is already off
when it is called.
So addtional waiting vblank is not necessary any more.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Acked-by: Inki Dae <inki.dae@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>