Add debug flag to re-enable wl_drm

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

include/sway/server.h

@@ -156,6 +156,7 @@ struct sway_debug {
 	bool noatomic;         // Ignore atomic layout updates
 	bool txn_timings;      // Log verbose messages about transactions
 	bool txn_wait;         // Always wait for the timeout before applying
+	bool legacy_wl_drm;    // Enable the legacy wl_drm interface
 };
 
 extern struct sway_debug debug;