i965 gen6: Turn on transform feedback extension.

This patch advertises support for EXT_transform_feedback on Intel
Gen6.

Since transform feedback support is not completely finished yet, for
now we only advertise support for it when MESA_GL_VERSION_OVERRIDE is
3.0 or greater (since transform feedback is required by GL version
3.0).

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

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

@@ -104,6 +104,9 @@ intelInitExtensions(struct gl_context *ctx)
       ctx->Const.GLSLVersion = 120;
    _mesa_override_glsl_version(ctx);
 
+   if (intel->gen == 6 && override_version >= 30)
+      ctx->Extensions.EXT_transform_feedback = true;
+
    if (intel->gen >= 5)
       ctx->Extensions.EXT_timer_query = true;