glamoregl: Allow glamor to be initialized on llvmpipe if a GPUDevice is present

This allows an UDL device to be automatically set up with GPU
acceleration via reverse PRIME.

    # DISPLAY=:0.2 xrandr --listproviders
    Providers: number : 2
    Provider 0: id: 0xec cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 1 outputs: 1 associated providers: 1 name:modesetting
    Provider 1: id: 0x12c cap: 0xb, Source Output, Sink Output, Sink Offload crtcs: 2 outputs: 2 associated providers: 1 name:Intel

Signed-off-by: Zoltán Böszörményi <zboszor@gmail.com>
(cherry picked from commit 24f69d2b)
3 jobs for fix-gpu-screen-assignment-1.20 in 6 minutes and 25 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Docker Image
passed debian-testing #11139002

00:00:31

 
  Build And Test
passed autotools-build-and-test #11139003

00:05:54

passed meson-build-and-test #11139004

00:00:59