drm: make legacy support code optional

[Dave Airlie]

This series attempts to make the legacy support core code removalable
at build time, if no drivers require it.

It reduces code size of the core drm.ko by ~10%.

It's also available in 
https://cgit.freedesktop.org/~airlied/linux/log/?h=drm-legacy-cleanup

The first patch has Daniels r-b on it, but I've moved DRM_VM selection
in the v2. The rest just move code around first, then the last 3 actually
make the legacy code removable.

Dave.