etnaviv: drm: fix check if BO is on a deferred destroy list

list_is_linked() isn't the right function to use in order to check if
the BO is on a cache bucket or the zombie list, as this checks if the
next pointer of the list isn't NULL. This is always the case with the
BO list item as it's always initialized, so the next pointer points to
the list head itself when the BO isn't on any list.

Use list_is_empty() to check if the BO is actually linked into one
of the deferred destroy lists.

Fixes: 1b1f8592 ("etnaviv: drm: properly handle reviving BOs via a lookup")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <mesa/mesa!20940>
(cherry picked from commit 196882a1)