Delete pending list after page flip instead of before

Under certain circumstances the currently displayed frame buffer can be
on the pending deletion list.  If this is deleted then the current crtc
will have no frame buffer and all further attempts to flip buffers will fail.
Fix management of pending deletion list

Fixes a use-after-free.
armsoc_bo_del() frees the list entry, but on next iteration of the
loop, xorg_list_for_each_entry reads the next entry pointer from there.
Use the _safe variant to avoid this.