drm/nouveau: initialize chan->fence.lock before use

Fence lock needs to be initialized before any call to nouveau_channel_put
because it calls nouveau_channel_idle->nouveau_fence_update which uses
fence lock.

BUG: spinlock bad magic on CPU#0, test/24134
 lock: ffff88019f90dba8, .magic: 00000000, .owner: <none>/-1, .owner_cpu: 0
Pid: 24134, comm: test Not tainted 3.0.0-nv+ #800
Call Trace:
 spin_bug+0x9c/0xa3
 do_raw_spin_lock+0x29/0x13c
 _raw_spin_lock+0x1e/0x22
 nouveau_fence_update+0x2d/0xf1
 nouveau_channel_idle+0x22/0xa0
 nouveau_channel_put_unlocked+0x84/0x1bd
 nouveau_channel_put+0x20/0x24
 nouveau_channel_alloc+0x4ec/0x585
 nouveau_ioctl_fifo_alloc+0x50/0x130
 drm_ioctl+0x289/0x361
 do_vfs_ioctl+0x4dd/0x52c
 sys_ioctl+0x42/0x65
 system_call_fastpath+0x16/0x1b

It's easily triggerable from userspace.

Additionally remove double initialization of chan->fence.pending.
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_channel.c

@@ -158,6 +158,7 @@ nouveau_channel_alloc(struct drm_device *dev, struct nouveau_channel **chan_ret,
 	INIT_LIST_HEAD(&chan->nvsw.vbl_wait);
 	INIT_LIST_HEAD(&chan->nvsw.flip);
 	INIT_LIST_HEAD(&chan->fence.pending);
+	spin_lock_init(&chan->fence.lock);
 
 	/* setup channel's memory and vm */
 	ret = nouveau_gpuobj_channel_init(chan, vram_handle, gart_handle);

drivers/gpu/drm/nouveau/nouveau_fence.c

@@ -539,8 +539,6 @@ nouveau_fence_channel_init(struct nouveau_channel *chan)
 			return ret;
 	}
 
-	INIT_LIST_HEAD(&chan->fence.pending);
-	spin_lock_init(&chan->fence.lock);
 	atomic_set(&chan->fence.last_sequence_irq, 0);
 	return 0;
 }