drm/ttm: Fix unused variable warning

[Alex Deucher]

On Fri, Feb 3, 2017 at 12:23 PM, Kent Russell <kent.russell at amd.com> wrote:
> This cleans up a remnant from Christian's patch (drm/ttm: revert "add
> optional LRU removal callback v2")
>
>  Kent
>

FWIW,  I already squashed the same fix into the patch when it went
upstream.  Feel free to apply to the internal trees however.

Reviewed-by: Alex Deucher <alexander.deucher at amd.com>