drm/amdgpu: Add CM_TEST_DEBUG regs for DCN

We'd like to use them for reading DCN debug status.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>