Building hw-display-virtio-*-gl modules with empty source set
Source files are added to hw-display-virtio-*-gl module source set after several conditions are met. However, module is built even if there's no source file added source set.
For example: Let's have CONFIG_VIRTIO_GPU and opengl enabled but have virgl disabled. In such case nothing is added to virtio-gpu-gl source set module hw-display-virtio-gpu-gl is build without virtio-gpu-gl-device support.
This is valid for all -gl variants (virtio-gpu-gl, virtio-gpu-pci-gl, virtio-vga-gl).
I don't think these modules should be build in such case.