compositor-drm: Retry atomic modeset if we get EBUSY

When hotplugging DP hubs with displays already connected quite often we
get udev event and start setting the modes before the driver is really
ready for the modeset.

This results in drmModeAtomicCommit failing with EBUSY.

Let's keep retying for 5s before giving up and messing up the
environment for the user.

Signed-off-by: Arkadiusz Hiler <arek@hiler.eu>
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