drm/nouveau/disp: Use NULL for pointers

The return type of exec_lookup() is struct nvkm_output *, so it should
return NULL rather than 0.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>