panfrost: Assign primitive_size.pointer only if writes_point_size() returns true

panfrost: Assign primitive_size.pointer only if writes_point_size() returns true

Checking vs->writes_point_size is not enough, as we might have a vertex
shader writing point size, but a primitive that's not MALI_POINT. That
currently works because emit_varying_descriptor() is called before the
primitive_size.constant field is update, but let's make the logic more
robust, just in case things are re-ordered at some point.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4083>