amdgpu: add a test for amdgpu_cs_wait_fences

Signed-off-by: monk.liu <Monk.Liu@amd.com>
[v2: actually hook up the test case]
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>