drm/nouveau: prime: fix refcount underflow

Calling nouveau_bo_ref() on a nouveau_bo without initializing it (and
hence the backing ttm_bo) leads to a refcount underflow.

Instead of calling nouveau_bo_ref() in the unwind path of
drm_gem_object_init(), clean things up manually.

Fixes: ab9ccb96 ("drm/nouveau: use prime helpers")
Reviewed-by: Ben Skeggs <bskeggs@nvidia.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240718165959.3983-2-dakr@kernel.org