drm/sched: Fix drm_sched_fence_free() so it can be passed an uninitialized fence

drm_sched_job_cleanup() will pass an uninitialized fence to
drm_sched_fence_free(), which will cause to_drm_sched_fence() to return
a NULL fence object, causing a NULL pointer deref when this NULL object
is passed to kmem_cache_free().

Let's create a new drm_sched_fence_free() function that takes a
drm_sched_fence pointer and suffix the old function with _rcu. While at
it, complain if drm_sched_fence_free() is passed an initialized fence
or if drm_sched_fence_free_rcu() is passed an uninitialized fence.

Fixes: dbe48d03 ("drm/sched: Split drm_sched_job_init")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210903120554.444101-1-boris.brezillon@collabora.com