drm/amdgpu: Fix a NULL deref in amdgpu_vm_add_prt_cb()

We accidentally dereference "cb" if the kmalloc() fails.

Fixes: 451bc8eb8fe6 ("drm/amdgpu: fix PRT teardown on VM fini v3")
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c

@@ -1528,7 +1528,7 @@ static void amdgpu_vm_add_prt_cb(struct amdgpu_device *adev,
 		if (fence)
 			dma_fence_wait(fence, false);
 
-		amdgpu_vm_prt_put(cb->adev);
+		amdgpu_vm_prt_put(adev);
 	} else {
 		cb->adev = adev;
 		if (!fence || dma_fence_add_callback(fence, &cb->cb,