drm/nouveau: nouveau_fence: Standardize list iterations

nouveau_fence.c iterates over lists in a non-canonical way. Since the
operations done are just basic for-each-loops and list-empty checks,
they should be written in the standard form.

Use standard list operations.

Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250424130254.42046-3-phasta@kernel.org


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