anv: use the null surface with unused push descriptor binding table entries

anv: use the null surface with unused push descriptor binding table entries

Some binding table entries have been identify as unused in the shaders
by the push constant analysis pass. We can just put the null entry in
there.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: b49b18f0b7 ("anv: reduce BT emissions & surface state writes with push descriptors")
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20555>
(cherry picked from commit 2d627f28c8fb55336d001846cfe70877b446d978)