drm/amdgpu: fix VM clearing for the root PD

drm/amdgpu: fix VM clearing for the root PD

[ Upstream commit 8604ffcbf04f8f4f3f55a9e46e5ff948b2ed4290 ]

We need to figure out the address after validating the BO, not before.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>