nir/lower_clip_cull: Fix an incorrect assert

Copy+paste error.  It was supposed to test cull and not clip.

Fixes: 4e69fba5 "nir: Rewrite lower_clip_cull_distance_arrays..."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109717


Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit f98fd9d1)