draft: backends/drm: support the audio property

Draft because it needs to be plumbed through kscreen.

It's also yet another undocumented drm property... it seems to control the audio state on some HDMI connectors, plus whether or not the port operates in DVI mode or not, but I'd like to know a it more. It's needed as a workaround for some display or driver woes: https://www.reddit.com/r/kde/comments/1be2a3f/alternative_to_xrandr_dvi_mode_on_wayland_kwin/