i965: Fix shadow batches to be the same size as the real BO.

brw_bo_alloc may round up our allocation size to the next bucket size.
In this case, we would malloc a shadow buffer that was the original
intended size, but use bo->size (the larger size) for all of our checks.

This could cause us to run off the end of the shadow buffer.

v2: Actually use the new BO size (caught by Lionel)

Reported-by: James Xiong <james.xiong@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: c7dcee58 (i965: Avoid problems from referencing orphaned BOs after growing.)
(cherry picked from commit da25ae92)