drm/amdgpu: avoid restore process run into dead loop.

In restore process worker, pinned BO cause update PTE fail, then
the function re-schedule the restore_work. This will generate dead loop.

Signed-off-by: gaba <gaba@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>