drm/ttm: use the new iterator in ttm_bo_flush_all_fences

This is probably a fix since we didn't even grabed a reference to the
fences.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-11-christian.koenig@amd.com