dri3: Always flush glamor before sharing pixmap storage with clients

Even if glamor_gbm_bo_from_pixmap / glamor_fd_from_pixmap themselves
don't trigger any drawing, there could already be unflushed drawing to
the pixmap whose storage we share with a client.
12 jobs for dri3_fd_from_pixmap-glamor-always-flush in 1 minute and 16 seconds (queued for 1 second)