pan/cs: Make sure we're not passed cs_now() to asynchronous instructions

FLUSH_CACHE2 is both deferrable and asynchronous. That means we should
always pass an explicit slot to signal instead of picking zero as we do
in cs_now().

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Rebecca Mckeever <rebecca.mckeever@collabora.com>
Reviewed-by: Mary Guillemard <mary.guillemard@collabora.com>
Reviewed-by: Lars-Ivar Hesselberg Simonsen <lars-ivar.simonsen@arm.com>
Reviewed-by: John Anthony <john.anthony@arm.com>
Acked-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30969>