backends/drm: remove support for transforming the scene with hardware rotation

Merged Xaver Hugl requested to merge work/zamundaaa/drop-hw-rotation into master

This feature is no longer needed, as the scene can efficiently do its own transformations now

This also means that the same thing can't be used to enable direct scanout with screen rotations anymore, but I think that's ok because

  • the current code is not well suited for that purpose
  • hardware rotation is still not enabled by default
  • the current code makes setting up displays more complicated, which is already more than complicated enough on its own
  • I'll implement using hardware rotation for direct scanout properly soon™

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