compositor: Move weston-launch socket lookup to drm backend

This is the only backend that uses the launcher, so let's set it
up there.

src/compositor-drm.c

@@ -2468,6 +2468,8 @@ drm_compositor_create(struct wl_display *display,
 	}
 
 	/* Check if we run drm-backend using weston-launch */
+	ec->base.launcher_sock =
+		weston_environment_get_fd("WESTON_LAUNCHER_SOCK");
 	if (ec->base.launcher_sock == -1 && geteuid() != 0) {
 		weston_log("fatal: drm backend should be run "
 			   "using weston-launch binary or as root\n");

src/compositor.c

@@ -2873,7 +2873,6 @@ weston_compositor_init(struct weston_compositor *ec,
 	wl_signal_init(&ec->update_input_panel_signal);
 	wl_signal_init(&ec->seat_created_signal);
 	wl_signal_init(&ec->output_created_signal);
-	ec->launcher_sock = weston_environment_get_fd("WESTON_LAUNCHER_SOCK");
 
 	ec->output_id_pool = 0;