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>
5 jobs for xwayland-multiplanar-fx in 1 minute and 58 seconds (queued for 2 seconds)
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