radv: fix a synchronization issue with primitives generated query on RDNA1-2

radv: fix a synchronization issue with primitives generated query on RDNA1-2

Only RDNA1-2 are affected because RADV needs to handle the legacy vs
NGG path for this query, and the NGG results are stored with 2 extra
64-bit values.

Fixes flakes with
dEQP-VK.transform_feedback.primitives_generated_query.* since VKCTS
1.3.7.0.

Cc: mesa-stable
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25862>
(cherry picked from commit 7d96fe853e1b007293054645caf54ade77a77c90)