platforms/drm: consider KWIN_DRM_DEVICES for hotplugged gpus
Effectively a stop-gap solution until libraries/plasma-wayland-protocols!17 is finished and implemented.
For VFIO some people are using the env var to have KWin not use a GPU, so that they can switch between using the gpu with a VM and Linux native apps. Once the VM releases it though, KWin opens the fd and if an output is connected it can't be released from the system again.