drm/nv50-nvc0: request and wait on notification of modeset completion

This should prevent a number of races from occuring, the most obvious of
which will be exposed when we start making use of the "display sync" evo
channel for page flipping.  The DS channel will reject any command stream
that doesn't completely agree with the current "master" state.

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