iris: Initialize bo->index to -1 when importing buffers

iris: Initialize bo->index to -1 when importing buffers

A value of -1 means that the buffer has never been used in an execbuf
buffer list in any of our contexts.  While setting this isn't critical,
doing so will allow us to short-circuit some looping in the next patch.

Cc: mesa-stable
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26806>
(cherry picked from commit d178334d5cafa9b215d38c7c443c4dbc168e66c5)