intel: Add an interface for saving/restoring the batchbuffer state.

intel: Add an interface for saving/restoring the batchbuffer state.

This will be used to avoid the prepare() step in the i965 driver's
state setup.  Instead, we can just speculatively emit the primitive
into the batchbuffer, then check if the batch is too big, rollback and
flush, and replay the primitive.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Paul Berry <stereotype441@gmail.com>