drm/cirrus: fix connector leak at unload

This fixes an '*ERROR* connector VGA-2 leaked!' splat at driver unload.
Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190111140242.29002-1-robdclark@gmail.com

[ kraxel: adapt to commit "c2d88e06 drm: Move the legacy kms
          disable_all helper to crtc helpers" ]
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/cirrus/cirrus_fbdev.c

@@ -256,6 +256,8 @@ static int cirrus_fbdev_destroy(struct drm_device *dev,
 {
 	struct drm_framebuffer *gfb = gfbdev->gfb;
 
+	drm_helper_force_disable_all(dev);
+
 	drm_fb_helper_unregister_fbi(&gfbdev->helper);
 
 	vfree(gfbdev->sysram);