i965: Reuse batch decoder infrastructure rather than open coding it.

i965: Reuse batch decoder infrastructure rather than open coding it.

With the new callback, Jason's newer batch decoder infrastructure
should be able to do just as well as the old open coded INTEL_DEBUG=bat
handling, with much less code.  If there are any limitations, we'd like
to improve the common code rather than doing one-off hacks here.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>