i965 gen6: Turn on transform feedback extension unconditionally.

Previously, we only enabled transform feedback when
MESA_GL_VERSION_OVERRIDE was 3.0 or greater, since transform feedback
support was not completely finished, so it didn't make sense to
advertise support for it unless absolutely necessary.

Now that transform feedback is fully implemented on gen6, we can
enable this extension unconditionally.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/intel/intel_extensions.c

@@ -104,7 +104,7 @@ intelInitExtensions(struct gl_context *ctx)
       ctx->Const.GLSLVersion = 120;
    _mesa_override_glsl_version(ctx);
 
-   if (intel->gen == 6 && override_version >= 30)
+   if (intel->gen == 6)
       ctx->Extensions.EXT_transform_feedback = true;
 
    if (intel->gen >= 5)