drm/nouveau/gem: remove (now) unneeded pre-validate fence sync

Now that nouveau_bo.c can handle sync when it actually needs to, we can
remove this and avoid a double semaphore acquire when syncing in the
command submission path.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

diff --cc drivers/gpu/drm/nouveau/nouveau_gem.c
Simple merge