drm/amdgpu: enable 8 SDMA instances for Arcturus

All the 8 SDMA instances work fine on the latest Gopher build model.

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