i965: Set brw->batch.emit only #ifdef DEBUG.

i965: Set brw->batch.emit only #ifdef DEBUG.

It's only used inside #ifdef DEBUG. Cuts ~1.7k of .text, and more
importantly prevents a larger code size regression in the next commit
when the .used field is replaced and calculated on demand.

   text     data      bss      dec      hex  filename
4945468   195152    26192  5166812   4ed6dc  i965_dri.so before
4943740   195152    26192  5165084   4ed01c  i965_dri.so after

And surround the emit and total fields with #ifdef DEBUG to prevent
such mistakes from happening again.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>