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.

BUG: 427060
FIXED-IN: 5.24.4


(cherry picked from commit 4a7007cd)