drm/nouveau: Add strap_peek to debugfs

Since we already expose the vbios.rom file here, why not also expose the
strap_peek?

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