drm/amdgpu: fix tear down order in amdgpu_vm_pt_free

When freeing PD/PT with shadows it can happen that the shadow
destruction races with detaching the PD/PT from the VM causing a NULL
pointer dereference in the invalidation code.

Fix this by detaching the the PD/PT from the VM first and then
freeing the shadow instead.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: https://gitlab.freedesktop.org/drm/amd/-/issues/2867
Cc: <stable@vger.kernel.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>