drm/nouveau/fbcon: fix NULL pointer access in nouveau_fbcon_destroy

When the fbcon object is initialized, but nouveau_fbcon_create is not
called, we run into a NULL pointer access within nouveau_fbcon_create when
unloading nouveau.

The call to drm_fb_helper_funcs.fb_probe is deferred until there is a
display for real since 4.14, that's why fbcon->helper.fb is still not set.

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>