xwayland: don't use GBM_BO_USE_SCANOUT

This flag should only be used when the caller intends to display the
buffer on a hardware plane. Xwayland isn't a DRM client, so it doesn't
make sense to use this flag.

This change will allow the driver to potentially use buffer parameters
that are more optimized.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
4 jobs for xwayland-no-scanout in 4 minutes and 43 seconds (queued for 1 second)