drm/amdgpu: fix new PD update code for Vega10 v2

We need to refer to the parent instead of the root BO for multi
level page tables on Vega10. Also don't set the PDE_PTE bit.

v2: Don't set the PDE_PTE bit either.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-and-Tested-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>