nvc0/ir: Only store viewport in scratch register for GP

nvc0/ir: Only store viewport in scratch register for GP

EMIT only applies to geometry shaders. For everything else, we want to
export the viewport normally.

Signed-off-by: Lyude <lyude@redhat.com>
Reviewed-by: Boyan Ding <boyan.j.ding@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>