radeonsi: mark glx-multithread-texture as a flake

This has apparently been flaking for a while, so let's mark it as such.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7772
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20004>

src/gallium/drivers/radeonsi/ci/gfx8-polaris11-fail.csv

@@ -4,7 +4,6 @@ glx@extension string sanity,Fail
 glx@glx-buffer-age vblank_mode=0,Fail
 glx@glx-make-glxdrawable-current,Fail
 glx@glx-multithread-makecurrent-1,Fail
-glx@glx-multithread-texture,Fail
 glx@glx-swap-event_async,Fail
 glx@glx-tfp,Fail
 glx@glx_arb_create_context_es2_profile@invalid opengl es version,Fail

src/gallium/drivers/radeonsi/ci/radeonsi-flakes.csv

@@ -1,5 +1,6 @@
 # piglit
 
+glx@glx-multithread-texture
 glx@glx-visuals-depth -pixmap
 glx@glx-visuals-stencil -pixmap
 spec@egl_mesa_configless_context@basic