drm/nouveau/disp: move head scanoutpos method

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

drm/nouveau/disp: collapse nv50_disp into nvkm_disp

Dump of one struct's members into another, with a couple of list
renames because of collisions.

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

drm/nouveau/disp: common implementation of scanoutpos method in nvkm_head

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drm/nouveau/disp: introduce object to track per-head functions/state

Primarily intended as a way to pass per-head state around during
supervisor handling, and share logic between NV50/GF119.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>