drm/nouveau: Make fence container helper usable driver-wide

drm/nouveau: Make fence container helper usable driver-wide

In order to implement a new DRM GPU scheduler callback in Nouveau, a
helper for obtaining a nouveau_fence from a dma_fence is necessary. Such
a helper exists already inside nouveau_fence.c, called from_fence().

Make that helper available to other C files with a more precise name.

Acked-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250710125412.128476-8-phasta@kernel.org