drm/nouveau/secboot: pass instance to LS firmware loaders

Having access to the secboot instance loading a LS firmware can be
useful to LS firmware handlers. At least more useful than just having an
out-of-context subdev pointer.

GP10B's firmware will also need to know the WPR address, which can be
obtained from the secboot instance.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>