drm/nouveau: Simplify nouveau_fence_done()

nouveau_fence_done() contains an if branch that checks whether a
nouveau_fence has either of the two existing nouveau_fence backend ops,
which will always evaluate to true.

Remove the surplus check.

Signed-off-by: Philipp Stanner <phasta@kernel.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20250424130254.42046-5-phasta@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>