drm/ttm: remove io_reserve_lru handling v3

That is not used any more.

v2: keep the NULL checks in TTM.
v3: remove unused variable

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/388646/