i965: Make a helper for recreating growing buffers.

i965: Make a helper for recreating growing buffers.

Now that we have two of these, we're duplicating a bunch of this logic.
The next commit will add more logic, which would make the duplication
seem worse.

This ends up setting EXEC_OBJECT_CAPTURE on the batch, which isn't
necessary (it's already captured), but it should be harmless.

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