radv: add support for writing layer/viewport index (v2)

radv: add support for writing layer/viewport index (v2)

This just adds the infrastructure to allow writing layer
and viewport index. It's just a first patch out of the geom
shader tree, and doesn't do much on its own.

v2: add missing if statement change (Bas)

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>