i965: Track last location of bo used for the batch

i965: Track last location of bo used for the batch

Borrow a trick from anv, and use the last known index for the bo to skip
a search of the batch->exec_bo when adding a new relocation. In defence
against the bo being used in multiple batches simultaneously, we check
that this slot exists and points back to us.

v2: Also update brw_batch_references()
v3: Reset bo->index on creation (Daniel)
v4: Improved explanation of bo->index (Kenneth)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>