drm/nouveau: wait for moving fence after pinning v2

We actually need to wait for the moving fence after pinning
the BO to make sure that the pin is completed.

v2: grab the lock while waiting

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
References: https://lore.kernel.org/dri-devel/20210621151758.2347474-1-daniel.vetter@ffwll.ch/
CC: stable@kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210622114506.106349-1-christian.koenig@amd.com