drm/amdgpu: disable sienna chichlid UMC RAS

disable UMC RAS in lieu of stability issues on certain sku

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>