clean up unused variable:
1. ring_lru_list
2. ring_lru_list_lock

related-commit:
drm/amdgpu: remove ring lru handling
Signed-off-by: NKevin Wang <kevin1.wang@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>