drm/nouveau: stop using ttm_bo_wait

TTM is just wrapping core DMA functionality here, remove the mid-layer.
No functional change.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221125102137.1801-5-christian.koenig@amd.com