drm/nouveau/secboot: abstract LS firmware loading functions

The WPR and LSB headers, used to generate the LS blob, may have a
different layout and sizes depending on the driver version they come
from. Abstract them and confine their use to driver-specific code.

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