iris: disable dynamic VAO fastpath on GFX version 9
On my SKL GT2 machine this improves performance in:
- GLB27 Egypt offscreen by 37%
- GLB27 TRex offscreen by 22%
- gfxbench5 Manhattan offscreen by 1.75%
- gfxbench5 Manhattan31 offscreen by 1.9%
- gfxbench5 Aztec Ruins high by 2.3%
In Intel performance CI on GFX version 9 it improves performance in:
- gfxbench5 Manhattan offscreen by 1.65%
- gfxbench5 Aztec Ruins normal offscreen by 1.33%
Closes: #3277 (closed)
Fixes: 42842306 ("mesa,st/mesa: add a fast path for non-static VAOs")
Huge thanks go to @jekstrand for explaining where the issue actually is.