drm/nouveau: support more than one write fence in fenv50_wndw_prepare_fb

Use dma_resv_get_singleton() here to eventually get more than one write
fence as single fence.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Lyude Paul <lyude@redhat.com>
Cc: nouveau@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220321135856.1331-14-christian.koenig@amd.com