i965: Fix batch-last mode to properly swap BOs.

i965: Fix batch-last mode to properly swap BOs.

On pre-4.13 kernels, which don't support I915_EXEC_BATCH_FIRST, we move
the validation list entry to the end...but incorrectly left the exec_bo
array alone, causing a mismatch where exec_bos[0] no longer corresponded
with validation_list[0] (and similarly for the last entry).

One example of resulting breakage is that we'd update bo->gtt_offset
based on the wrong buffer.  This wreaked total havoc when trying to use
softpin, and likely caused unnecessary relocations in the normal case.

Fixes: 29ba502a4e28471f67e4e904ae503157087efd20 (i965: Use I915_EXEC_BATCH_FIRST when available.)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>