i965: Reorganize batch/state BO fields into a 'brw_growing_bo' struct.

i965: Reorganize batch/state BO fields into a 'brw_growing_bo' struct.

We're about to add more of them, and need to pass the whole lot of them
around together when growing them.  Putting them in a struct makes this
much easier.

brw->batch.batch.bo is a bit of a mouthful, but it's nice to have things
labeled 'batch' and 'state' now that we have multiple buffers.

Fixes: 2dfc119f22f257082ab0 "i965: Grow the batch/state buffers if we need space and can't flush."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103101
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>