xwayland/glamor/gbm: Don't close fence_fd after xwl_glamor_wait_fence
eglCreateSyncKHR takes ownership of the file descriptor. Noticed by inspection.
While we're at it, move the fence_fd declaration to the scope where it's used.
Last but not least, close the fd in xwl_glamor_wait_fence when bailing before calling eglCreateSyncKHR, and document that it takes ownership.