drm/mxsfb: Drop drm_vblank_cleanup

Almost right but still racy, it's called before the interrupts are
uninstalled. So let's just drop it.

Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-7-daniel.vetter@ffwll.ch

drivers/gpu/drm/mxsfb/mxsfb_drv.c

@@ -256,7 +256,6 @@ static void mxsfb_unload(struct drm_device *drm)
 
 	drm_kms_helper_poll_fini(drm);
 	drm_mode_config_cleanup(drm);
-	drm_vblank_cleanup(drm);
 
 	pm_runtime_get_sync(drm->dev);
 	drm_irq_uninstall(drm);