tgsi: use TGSI_INTERPOLATE_x arguments instead of zeros in ureg code

tgsi: use TGSI_INTERPOLATE_x arguments instead of zeros in ureg code

TGSI_INTERPOLATE_CONSTANT and TGSI_INTERPOLATE_LOC_CENTER have the
value zero so there's no change in behavior.  It seems funny to
declare these fs input registers with constant interpolation.  But
it looks like ureg_DECL_input_layout() is not called anywhere and
ureg_DECL_input() is only called from
util_make_geometry_passthrough_shader().

Reviewed-by: Mathias Fröhlich <mathias.froehlich@web.de>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>