drm/nouveau: use the new interator in nv50_wndw_prepare_fb

Makes the handling a bit more complex, but avoids the use of
dma_resv_get_excl_unlocked().

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-27-christian.koenig@amd.com