radv: fix binding transform feedback buffers

The mask should be accumulated if two calls are used for
binding two buffers at different indexes. Otherwise, the
driver only accounts for the last one.

Noticed while glancing at this code.

Cc: 18.3 19.0 <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 4fa61273)