drm/nouveau: fence: fix type cast warning in nouveau_fence_emit()

Fix the following warning.

  drivers/gpu/drm/nouveau/nouveau_fence.c:210:45: sparse: sparse:
  incorrect type in initializer (different address spaces)
  @@     expected struct nouveau_channel *chan
  @@     got struct nouveau_channel [noderef] __rcu *channel

We're just about to emit the fence, there is nothing to protect against
yet, hence it is safe to just cast __rcu away.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309140340.BwKXzaDx-lkp@intel.com/
Fixes: 978474dc

 ("drm/nouveau: fence: fix undefined fence state after emit")
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230916011501.15813-1-dakr@redhat.com