xwayland: Set GLVND driver based on GBM backend name

With the GBM backend becoming usable with different drivers such as
NVIDIA, set the GLVND vendor to the same value as the GBM backend name.

Mesa implementation however returns "drm" so we need to special case
this value - Basically, for anything other than "drm" we simply assume
that the GBM backend name is the same as the vendor.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: James Jones <jajones@nvidia.com>
Tested-by: James Jones <jajones@nvidia.com>
5 jobs for glvnd-gbm-backend in 6 minutes and 58 seconds
Status Name Job ID Coverage
  Docker Image
passed debian-buster #14419617

00:00:09

 
  Build And Test
passed autotools #14419618

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passed meson #14419619

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passed meson-noglamor #14419620

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passed mingw-cross-build #14419621

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