drm/amdgpu: lock the eviction fence for wq signals it

drm/amdgpu: lock the eviction fence for wq signals it

Lock and refer to the eviction fence before the eviction fence
schedules work queue tries to signal it.

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Prike Liang <Prike.Liang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Arvind Yadav <Arvind.Yadav@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>