backends/drm: fall back to legacy mode in virtual machines

Virtual machines aren't properly supporting atomic mode setting yet, which causes the cursor to be offset, and will cause more issues with overlay planes. In order to prevent that from impacting users, fall back to legacy, unless KWIN_DRM_NO_AMS is set (to 0).

While I really don't like patching around what I think is very clearly upstream bugs, we've worked around some similar-ish age-old (and new, in the case of NVidia) upstream bugs already, adding one more isn't a big harm in this simple case. I'll make sure it gets its proper discussion in the relevant upstream mailing lists once I have time for that.

BUG: 427060 FIXED-IN: 5.24.4