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

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.

This is a pretty big performance hit for vertex shaders.  Using glmark2 -b
build:use-vbo=true:

i915g-vbuf:     211 fps
i915g-nonvbuf:  185 fps
i915c:          41 fps

Given how massively better i915g still is than i915c (llvmpipe VS instead
of the classic swrast interpreter), I think it's still worth it to get
i915g correct before we fix this perf regression.

Fixes: #4971
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13052>