drm/nouveau: add instmem flush() hook

drm/nouveau: add instmem flush() hook

This removes the previous prepare_access() and finish_access() hooks, and
replaces it with a much simpler flush() hook.

All the chipset-specific code before nv50 has its use removed completely,
as it's not required there at all.

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