drm/ttm: Fix potential ttm_mem_evict_first races.

1) The function was previously called with a potentially empty
LRU list which would have lead to an OOPS or servere corruption.
2) In rare cases, after reservation has succeeded, another process may
already have evicted it or even pinned it. We must revalidate the
buffer status after releasing the lru lock.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>