vk/cmd_buffer: Rework validate list creation

vk/cmd_buffer: Rework validate list creation

The algorighm we used previously required us to call add_bo in a particular
order in order to guarantee that we get the initial batch buffer as the
last element in the validate list.  The new algorighm does a recursive walk
over the buffers and then re-orders the list.  This should be much more
robust as we start to add circular dependancies in the relocations.