Don't use xorg_list_for_each_entry_safe for signalled flips

drm_wait_pending_flip can get called from drm_handle_event, in which
case xorg_list_for_each_entry_safe can end up processing the same entry
in both. To avoid this, just process the first list entry until the list
is empty.

(Ported from amdgpu commit 26770be44b89b83bf39c28f2fe284c8cb92ed0c0)