modesetting: Indirect the glamor API through LoaderSymbol

Prerequisite for building all of xserver with -z now.

Gitlab: xorg/xserver#692
3 jobs for modesetting-z-now in 7 minutes and 47 seconds (queued for 15 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-testing #940322

00:00:37

 
  Build And Test
passed autotools-build-and-test #940323

00:05:39

passed meson-build-and-test #940324

00:01:30