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

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

Fixes: 451bc8eb ("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>