mesa/teximage: use correct extension for accept stencil texture.
This was using the wrong extension, ARB_stencil_texturing doesn't mention any changes in this area. Fixes "dEQP-GLES3.functional.fbo.completeness.renderable.texture. stencil.stencil_index8." Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90751 Signed-off-by:Dave Airlie <airlied@redhat.com> Reviewed-by:
Ilia Mirkin <imirkin@alum.mit.edu> Reviewed-by:
Kenneth Graunke <kenneth@whitecape.org>