drm/nouveau/disp,pm: constify nvkm_object_func structures

These nvkm_object_func structures are never modified.  All other
nvkm_object_func structures are declared as const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>