i915g: Use the non-vbuf code path by default to fix index overflows.

We were assertion failing on some large draws due to indices >16bits, despite asking draw to limit the max indices. I haven't managed to track it down, so flip us back to the older, non-index drawing path that doesn't hit this bug until it can get fixed. Leave an I915_DEBUG=vbuf flag around so we can look into this later.

Fixes: #4971 (closed)