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

Fixes: 451bc8eb ("drm/amdgpu: fix PRT teardown on VM fini v3")
Reviewed-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NHarry Wentland <harry.wentland@amd.com>
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>