egl/sl: also allow virtgpu to fallback to kms_swrast

virtio-gpu fallbacks to software rendering when 3D features
are unavailable since 6c5ab, and kms_swrast is more
feature complete than swrast.

v2: Add comment (Emil)

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
1 file changed