drm/nouveau/disp: collapse nv50_disp_func into nvkm_disp_func

Aside from a chicken-and-egg problem with a duplicate 'root' member,
this is a straight dump of function pointers from one struct into
another.

The left-over wrapping mess in >=nv50 structs will be fixed later.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>