Use standard list_head double-linked list and related iterators,
helpers, etc, rather than weird combo of instruction array and next
pointers depending on stage.  Now block has an instrs_list.  In
certain stages where we want to remove and re-add to the blocks list
we just use list_replace() to copy the list to a new list_head.

Signed-off-by: Rob Clark <robclark@freedesktop.org>