drm/amdgpu: support gc v9_4_3 ring_test running on all xcc

Each xcc has its own sratch_reg offset

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Le Ma <Le.Ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>