drm/moc: Mark legacy fields in drm_driver as such

No point in documenting these, they only confuse.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-13-daniel.vetter@ffwll.ch