iris: Don't assume UBO indices are constant

It will be true for the constant/system value buffer because they use a
constant zero but it's not true in general.  If we ever got here when
the source wasn't constant, nir_src_as_uint would assert.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 9dc57eeb)