glamor_egl: handle fd export failure in glamor_egl_fds_from_pixmap

Check the fd for validity before giving a success return code.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Simon Ser <contact@emersion.fr>
Tested-by: Guido Günther <agx@sigxcpu.org>