glamor: Don't require the glamoregl module to be pre-loaded with xserver >= 1.15

glamor: Don't require the glamoregl module to be pre-loaded with xserver >= 1.15

Inspired by (ok, copied from):

xf86-video-ati commit f2a0a5cf6c5a21e2a02280e110a4eb8e6609dace
Author: Michel Dänzer <michel.daenzer@amd.com>
Date:   Wed Jan 22 11:04:42 2014 +0900

    Don't require the glamoregl module to be pre-loaded with xserver >= 1.15

    The issues with loading it on demand have been fixed in xserver 1.15.

    Inspired by Jérôme Glisse on IRC.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>