st/nir: fix arb fragment stage conversion

The comment even justifies the wrongness wrongly.

We should be translating to pipe values properly here or else
fragment maps to tess ctrl.

Fixes: 3d7611e9 ("st/nir: use NIR for asm programs")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 365f2470)