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>

drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c

@@ -1602,7 +1602,7 @@ static int sdma_v4_0_early_init(void *handle)
 	if (adev->asic_type == CHIP_RAVEN)
 		adev->sdma.num_instances = 1;
 	else if (adev->asic_type == CHIP_ARCTURUS)
-		adev->sdma.num_instances = 2;
+		adev->sdma.num_instances = 8;
 	else
 		adev->sdma.num_instances = 2;