wayland/dma-buf: Handle lack of gbm_device gracefully

In some configurations (e.g. NVIDIA driver 470) Xwayland may use DMA buffer for passing buffers around. When this is done, we might attempt to scanout these buffers when they are fullscreen, and to do so we import them using gbm.

However, for the mentioned configuration, there is no gbm device available for importing. This was not handled, and resulted in a crash; avoid this crash by checking whether we have a gbm device and fail gracefully if we don't.

Closes: #2098 (closed)