glamor_egl: properly get FDs from multiplanar GBM BOs

Multiplanar GBM buffers can point to different objects from each plane.
Use the _for_plane API when possible to retrieve the correct prime FD
for each plane.

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