drm/ttm: fix busy reference in ttm_mem_evict_first

The busy BO might actually be already deleted,
so grab only a list reference.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thellstrom@vmware.com>
Link: https://patchwork.freedesktop.org/patch/332877/