drm/amdgpu: add register distance members into vmhub structure

This patch is to abstract register distances between two continuous
context domains and invalidation engines. In different ip headers, these
distances may be differences.

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>