drm: don't double-free on driver load error

All instances of drm_dev_register are followed by drm_dev_free on
failure. Don't free dev->control/render/primary on failure, as they will
be freed by drm_dev_free since commit 8f6599da (drm: delay minor
destruction to drm_dev_free()). Instead unplug them.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reported-and-tested-by: Bruno Prémont <bonbons@linux-vserver.org>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>