intel/aubinator: Properly handle batch buffer chaining

intel/aubinator: Properly handle batch buffer chaining

The original aubinator that Kristian wrote had a bug in the handling of
MI_BATCH_BUFFER_START that propagated into the version in upstream mesa.
In particular, it ignored the "2nd level" bit which tells you whether this
MI_BATCH_BUFFER_START is a subroutine call (2nd level) or a goto.  Since
the Vulkan driver uses batch chaining, this can lead to a very confusing
interpretation of the batches.  In some cases, depending on how things are
laid out in the virtual GTT, you can even end up with infinite loops in
batch processing.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>