meson: allow building all glx without any drivers

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111016
Fixes: a47c525f
       ("meson: build glx")
10 jobs for !1222 with glx-dri-without-drivers in 9 minutes and 16 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #392343
debian

00:00:27

 
  Build+Test
passed #392345
meson-clang

00:07:37

passed #392349
meson-clover

00:08:48

passed #392348
meson-main

00:06:01

passed #392344
meson-swr-glvnd

00:06:09

passed #392350
meson-vulkan

00:02:40

passed #392352
scons-llvm

00:04:01

passed #392351
scons-nollvm

00:03:52

passed #392346
scons-swr

00:05:59

passed #392347
scons-win64

00:06:39