radeonsi/nir: fix number of used samplers

radeonsi/nir: fix number of used samplers

Commit f3e978db incorrectly assumed the maximum number of
samplers was equal to the max number of defined samplers
e.g. where bindings skip slots.

This fixes an assert in si_nir_load_sampler_desc() for an
enemy territory quake wars shader. And fixes potential bugs with
incorrect bounds limiting in the same code for production builds
of mesa.

Fixes: f3e978db ("radeonsi/nir: Remove uniform variable scanning")
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>