backends/drm: enable hardware rotation by default

KWin should now always gracefully fall back to software rotation if the requested rotation can't be done in hardware.

For testing I used my laptop where it worked fine, and then also made KWin simulate failing atomic tests whenever hardware transforms were requested, and it worked fine with that as well.

This is best paired with !2249 (merged) but it should work on a lot of hardware without it, too.