drm: delay minor destruction to drm_dev_free()

Instead of freeing minors in drm_dev_unregister(), we only unplug them and
delay the free to drm_dev_free(). Note that if drm_dev_register() has
never been called, minors are NULL and this has no effect.

This change is needed to allow early device unregistration. If we want to
call drm_dev_unregister() on live devices, we need to guarantee that
minors are still valid (but unplugged). This way, any open file can still
access file_priv->minor->dev to get the DRM device. However, the minor is
unplugged so no new users can occur.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>