drm/amdgpu: finish wiring up sid.h in DCE6

For coherence with DCE8 et DCE10, add or move some values under sid.h
and remove duplicated from si_enums.h.

Signed-off-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>