radv: fix GDS counters with NGG streamout
On GFX10.3.
$ RADV_PERFTEST=ngg_streamout ./deqp-vk -n dEQP-VK.transform_feedback.*
Test run totals:
Passed: 7420/9208 (80.6%)
Failed: 0/9208 (0.0%)
Not supported: 1788/9208 (19.4%)
\o/