backends/drm: retry opening drm nodes for up to 5s if it fails with EBUSY

Xaver Huglrequested to merge
work/zamundaaa/drm-master into master

This can sometimes happen when switching between drm masters: While KWin is getting started, another process is still drm master, and so opening the node fails. This seems to be a really racy process, so just re-try opening the node for up to 5 seconds to work around the issue.

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