drm/sman: rip out owner tracking

In contrast to kms drivers, sis/via _always_ associated a buffer with
a drm fd. So by the time we reach lastclose, all open drm fds are gone
and with them their associated objects.

So when sis/via call drm_sman_cleanup in their lastclose funcs, that
will free 0 objects.

The owner tracking now serves no purpose at all, hence rip it ou. We
can't kill the corresponding fields in struct drm_memblock_item yet
because we hijack these in the new driver private owner tracking. But
now that drm_sman.c doesn't touch ->owner_list anymore, we need to
kill the list_move hack and properly add the item to the file_priv
list.

Also leave the list_del(&obj->owner_list) in drm_sman_free for the
moment, it will move to the drivers when sman disappears completely.

v2: Remove the redundant INIT_LIST_HEAD as noted by Chris Wilson

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>