drm/amdgpu: clean up unused variable about ring lru

clean up unused variable:
1. ring_lru_list
2. ring_lru_list_lock

related-commit:
drm/amdgpu: remove ring lru handling

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>