drm/amd/amdgpu: remove redundant flush_delayed_work

When using cancel_delayed_work_sync, there's no need
to flush_delayed_work first. This sequence can lead to
a redundant loop of work executing.

Signed-off-by: Jingwen Chen <Jingwen.Chen2@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>