drm/amdgpu: only use one gfx pipe for Sienna_Cichlid

Only enable one gfx pipe for sienna_cichlid currently.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>