backend/drm: re-use previous mode blob if it didn't change

If the old mode is the same as the new one, we don't need to
re-create the mode blob. This happens when the compositor calls
wlr_output_set_mode() at startup, for instance.
3 jobs for drm-keep-mode-blob in 3 minutes and 2 seconds (queued for 3 seconds)
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