compositor(-drm): keep non-desktop displays turned off by default

Weston happily tries to burn its desktop wallpaper into any head mounted display it can find. To avoid this, use the DRM connector "non-desktop" property, mark weston_heads that represent HMDs and keep them turned off by default. They can still be enabled by setting an explicit [output] mode (or "mode=preferred") in weston.ini for the DRM backend.

I'm unsure about a few things:

• Should this wait for lease support?
• Should the change in simple_heads_changed be dropped? DRM is the only backend with the non-desktop property after all, and there would be no way to re-enable theoretical non-desktop displays for other backends.
• Should non_desktop be a property of weston_head at all, or should this be limited to drm_head, dropping weston_head_set_non_desktop() and replacing weston_head_is_non_desktop() with drm_head_is_non_desktop()?