drm/amdgpu: export test ib debugfs interface

As Christian and David's suggestion, submit the test ib ring debug interfaces.
It's useful for debugging with the command submission without VM case.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>