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: 29ba502a4e (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>

src/mesa/drivers/dri/i965/intel_batchbuffer.c

@@ -780,11 +780,16 @@ submit_batch(struct brw_context *brw, int in_fence_fd, int *out_fence_fd)
       } else {
          /* Move the batch to the end of the validation list */
          struct drm_i915_gem_exec_object2 tmp;
+         struct brw_bo *tmp_bo;
          const unsigned index = batch->exec_count - 1;
 
          tmp = *entry;
          *entry = batch->validation_list[index];
          batch->validation_list[index] = tmp;
+
+         tmp_bo = batch->exec_bos[0];
+         batch->exec_bos[0] = batch->exec_bos[index];
+         batch->exec_bos[index] = tmp_bo;
       }
 
       ret = execbuffer(dri_screen->fd, batch, brw->hw_ctx,