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