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>
(cherry picked from commit 5daf42b4)
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