panthor: use drm_gem_object.gpuva.lock instead of gpuva_list_lock

panthor: use drm_gem_object.gpuva.lock instead of gpuva_list_lock

Now that drm_gem_object has a dedicated mutex for the gpuva list that is
intended to be used in cases that must be fence signalling safe, use it
in Panthor.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250827-gpuva-mutex-in-gem-v3-2-bd89f5a82c0d@google.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>