drm/amdgpu: enable all 8 sdma instances for Arcturus silicon

The more 6 sdma instances work fine now with DF fix in vbios:
  * mmDF_PIE_AON_MiscClientsEnable(0x1c728)=0x3fe(DF_ALL_INSTANCE)
       [9:4]MmhubsEnable=3f (change from 0)

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>