Revert "platforms/drm: Introduce DrmPipeline"

This reverts commit 5a22deda.

We still need more work to finish the DrmPipeline. At the moment, there
are a few major issues, e.g. some outputs not turning on, output
transforms not working correctly, a crash when changing dpms mode.

Let's merge this change back once all major issues are fixed and after
more testing.