radv: emit correct last export when Z/stencil export is enabled

radv: emit correct last export when Z/stencil export is enabled

I was getting a random GPU hang in the renderpass simple tests,
it turns out sometimes radv emitted the wrong thing "last".

This fixes the logic to emit Z/stencil last if they occur,
and not mark a color output as last. Also this relies on the
Z/STENCIL being the first two fragment outputs, which they are
so yay.

Fixes: dEQP-VK.renderpass.simple.color_depth (random hangs)
Cc: "13.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>