radv: do not emit rbplus if attachments are undefined

Fixes some crashes with dEQP-VK.geometry.layered.*.secondary_cmd_buffer
on Raven and other chips that allow rbplus.

This just prevents a crash and rbplus probaby needs more work.

Cc: 19.2 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 956d825e)