anv: Don't include two different pack headers

The batch chain logic only needs the pre-gen8 size of
MI_BATCH_BUFFER_START, which seems like something we can make a special
case for. The other two gen7 references, MI_BATCH_BUFFER_END and
MI_NOOP, are the same on all gens.

Signed-off-by: Kristian H. Kristensen <hoegsberg@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>