7e69a7076f ("Drop wl_drm") has dropped wl_drm, however a lot of
software wasn't quite ready for this (Xwayland, libva, amdvlk).
Keep wl_drm disabled by default to pressure the wl_drm phase-out,
but add a -Dlegacy-wl-drm flag for users to restore the previous
behavior in the meantime.
References: https://github.com/swaywm/sway/issues/7897