drm/hisilicon: Fix ADE vblank on/off handling

Vblank turn on should be called in crtc's enable callback.
And turn off called in crtc's disable callback.

Thanks to Daniel Vetter, this bug is reported by him.

Reported-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160630092300.141864-1-xinliang.liu@linaro.org