backend/drm: only request page-flip if active

It doesn't make sense to request a page-flip for a disabled output.

Fixes: 84e727daae ("backend/drm: request page-flip event on modeset")
Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3528

backend/drm/drm.c

@@ -577,7 +577,7 @@ bool drm_connector_commit_state(struct wlr_drm_connector *conn,
 			return false;
 		}
 	}
-	if (pending.modeset) {
+	if (pending.modeset && pending.active) {
 		flags |= DRM_MODE_PAGE_FLIP_EVENT;
 	}