kmscube: check return value when creating GBM device

In the off-chance that gbm_create_device fails and returns a NULL value, further
down the line a segfault will be triggered when dereferencing gbm.dev.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
2 jobs for fix-init_gbm in 39 seconds (queued for 3 seconds)
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