drm/amdgpu: fix addr handling in amdgpu_vm_bo_update_mapping

Otherwise we will look at the wrong place in the IB when GART
mappings are split into smaller updates.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Tom StDenis <Tom.StDenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>