radv: add initial copy descriptor support. (v2)

It appears the latest dota2 vulkan uses this,
and we get a hang in VR mode without it.

v2: remove finishme I left in after finishing.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Andres Rodriguez <andresx7@gmail.com>
Cc: "17.2 17.3" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 4bcb48b8)