i965: Move brw_state_batch code to intel_batchbuffer.c

i965: Move brw_state_batch code to intel_batchbuffer.c

The batch buffer and state buffer code is fairly tied together,
and having it in one .c file will make refactoring easier.

Also, drop some commentary above brw_state_batch.  The "aperture
checking performance hacks" are long since gone, so that paragraph
makes little sense at this point.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>