drm/nouveau/secboot: plug memory leak in ls_ucode_img_load_gr() error path

The last goto looks spurious because it releases less resources than the
previous one.
Also free 'img->sig' if 'ls_ucode_img_build()' fails.

Fixes: 9d896f3e

 ("drm/nouveau/secboot: abstract LS firmware loading functions")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>