Ideas for how to handle the drm_device sharing between xe and i915-display

It's tricky and much potential for grossness. Roughly what we need:

• i915-display needs to be able to use the same struct when running both with xe and i915-gem
• that struct needs to contain a single struct drm_device
• there needs to be some ops table in that common struct so that i915-display is abstracted from the gem implementation, at least somewhat
• ideally we don't pollute i915-gem and xe-gem with struct definitions of the other driver