mesa: fix clip plane enable breakage

mesa: fix clip plane enable breakage

Broken by:

commit 00173d91b70ae4dcea7c6324ee4858c498cae14b
Author: Marek Olšák <marek.olsak@amd.com>
Date:   Sat Jun 10 12:09:43 2017 +0200

    mesa: don't flag _NEW_TRANSFORM for st/mesa if possible

It also optimizes the case slightly for GL core.

It doesn't try to fix that glEnable might be a bad place to do the
clip plane transformation.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>