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>
5 jobs for fix-gpu-screen-assignment in 5 minutes and 11 seconds (queued for 8 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #11138920

00:00:23

 
  Build And Test
passed autotools #11138921

00:04:47

passed meson #11138922

00:02:09

passed meson-noglamor #11138923

00:01:46

passed mingw-cross-build #11138924

00:01:12