virgl: Use minigbm_create_default_device helper to create gbm device

Applies only when ENABLE_MINIGBM_ALLOCATION is defined.

The helper was added in crrev.com/c/4163418. It looks for a DRM primary node when no render node can be found, making virgl work through llvmpipe on drivers with only a primary node, for example vkms.

Signed-off-by: Yi Xie yixie@google.com