dxil: Use nir_const_value_for_uint in dxil_nir_lower_int_samplers

dxil: Use nir_const_value_for_uint in dxil_nir_lower_int_samplers

This change should avoid any accidental rounding issues because of
border colors getting stored in a float in dxil_wrap_sampler_state.  It
also switches us to using the correct helpers for nir_const_value so we
can avoid any weird uninitialized data failures that can be caused by
filling out the fields in the struct directly.

Fixes: b9c61379ab4c ("microsoft/compiler: translate nir to dxil")
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19689>