backend/drm: fix missing cursor on external monitors with multi-GPU setup and nouveau

We need to make the multi-GPU surface current before the glFinish call.

Closes: https://github.com/swaywm/sway/issues/5319

backend/drm/drm.c

@@ -1026,6 +1026,7 @@ static bool drm_connector_set_cursor(struct wlr_output *output,
 	}
 
 	if (plane->cursor_enabled) {
+		drm_fb_acquire(&plane->pending_fb, drm, &plane->mgpu_surf);
 		/* Workaround for nouveau buffers created with GBM_BO_USER_LINEAR are
 		 * placed in NOUVEAU_GEM_DOMAIN_GART. When the bo is attached to the
 		 * cursor plane it is moved to NOUVEAU_GEM_DOMAIN_VRAM. However, this