drm/nouveau: stop using dma_resv_excl_fence

Instead use the new dma_resv_get_singleton function.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: nouveau@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220321135856.1331-6-christian.koenig@amd.com