drm/nouveau: prime: fix refcount underflow

In the Linux kernel, the following vulnerability has been resolved: 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. (cherry picked from commit 1b93f3e89d03cfc576636e195466a0d728ad8de5)