drm/nouveau/graph: pad firmware code at load time

Pad the microcode to a multiple of 0x40 words, otherwise firmware will
fail to run from non-prepadded firmware files.

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