zink: set multiviewport cap in ntv when gl_ViewportIndex is a written output

this cap varies based on the shader stage, but it must be enabled any time this
output is written

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6349>
1 file changed