drm/amdgpu: use register distance member instead of hardcode in mmhub v9.4

This patch updates to use register distance member instead of hardcode
in mmhub v9.4.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Tested-by: AnZhong Huang <anzhong.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>