[Intel-gfx] [RFC v5] refactor intel display a bit more.

[Dave Airlie]

Thanks Jani for cleaning up the last bits, I started from your patches
and changed things around a little.

This move the i915 plane code to i9xx_plane.[ch]
The later skl plane now moves to skl_universal_plane.[ch]

I think I've also taken Ville's suggestsion on board for that file,
and I've use --patience to generate the diffs.

I'm in agreement we should really consider landing thses rather than
overly bikeshedding at this point otherwise it'll become very hard
to get this moving forward.

Dave.