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.

Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Signed-off-by: Yi Xie <yixie@google.com>
Part-of: <virgl/virglrenderer!1034>