drm/nouveau: remove unnecessary null pointer check from nouveau_fence_new

the variable chan is dereferenced in line 190, so it is no reason to check
null again in line 198.

Signed-off-by: Cong Ding <dinggnu@gmail.com>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>