anv/batch_chain: Simplify secondary batch return chaining

anv/batch_chain: Simplify secondary batch return chaining

Previously, we did this weird thing where we left space and an empty
relocation for use in a hypothetical MI_BATCH_BUFFER_START that would be
added to the secondary later.  Then, when it came time to chain it into
the primary, we would back that out and emit an MI_BATCH_BUFFER_START.
This worked well but it was always a bit hacky, fragile and ugly.  This
commit instead adds a helper for rewriting the MI_BATCH_BUFFER_START at
the end of an anv_batch_bo and we use that helper for both batch bo list
cloning and handling returns from secondaries.  The new helper doesn't
actually modify the batch in any way but instead just adjusts the
relocation as needed.

Reviewed-by: Scott D Phillips <scott.d.phillips@intel.com>