We'd like to start using soft-pin to assign BO addresses up front, and
never move them again.  Our previous plan for dealing with 48-bit VF
cache bugs was to relocate vertex buffers to the low 4GB, so we'd never
have addresses that alias in the low 32 bits.  But that requires moving
buffers dynamically.

This patch tracks the last seen BO address for each vertex/index buffer,
and emits a VF cache invalidate if the high bits change.  (Ideally, we
won't hit this case very often.)  This should work for the soft-pin
case, but unfortunately won't work in the relocation case, as we don't
actually know the addresses.  So, we have to use both methods.

v2: Mention that the cache uses a <VertexBufferIndex, Address> tuple
    more explicitly (suggested by Scott).  Mention "single batch" too
    (suggested by Chris).

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