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 <>
Reviewed-by: Simon Ser <>
Tested-by: Guido Günther <>
5 jobs for xwayland-multiplanar-fx in 1 minute and 58 seconds (queued for 2 seconds)