drm/amdgpu: cleanup amdgpu_vm_validate_pt_bos v2

Use list_for_each_entry_safe here.

v2: Drop the optimization, it doesn't work as expected.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>