backend: disable DRM backend monitor when WLR_DRM_DEVICES is used

WLR_DRM_DEVICES specifies a static list of DRM devices to open
at startup.

Do not create DRM backends for hotplugged DRM devices when it's
set.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3595

backend/backend.c

@@ -233,7 +233,9 @@ static bool attempt_drm_backend(struct wl_display *display,
 		return NULL;
 	}
 
+	if (getenv("WLR_DRM_DEVICES") == NULL) {
 		drm_backend_monitor_create(backend, primary_drm, session);
+	}
 
 	return true;
 }