anv: fix invalid binding table index computation

The ++ operator strikes again.

Fixes: f92c5bc8 ("anv/device: fix maximum number of images supported")
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 4149d41f)