This changes splits up the monster file containing different types used
in the DRM backend into separate files per class:
- drm_backend.{h,cpp}
- drm_buffer.{h,cpp}
- drm_inputeventfilter.{h,cpp}
- drm_output.{h,cpp}
- drm_pointer.h
No actual code changes other than build fixes.
Clean up headers in the split files.