gallium/vl: fix compute tgsi shaders to not process undefined components

This caused nouveau's function handling logic to think that the MAIN
function was due to receive external parameters, and cascaded some
failures after that. Instead avoid having the undefined components in
the first place.

Fixes: f6ac0b5d (gallium/auxiliary/vl: Add compute shader to support video compositor render)
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111213
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111217


Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit face27fd)