drm/ttm: drop evicted from ttm_bo.

This was unused.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917064132.148521-3-airlied@gmail.com