drm/nouveau/dispnv50: Fix runtime PM ref tracking for non-blocking modesets

This is something that got noticed a while ago back when I was fixing a
large number of runtime PM related issues in nouveau, but never got
fixed:

https://patchwork.freedesktop.org/series/46815/#rev7



It's not safe to iterate the entire list of CRTCs in
nv50_disp_atomic_commit(), as we could be doing a non-blocking modeset
on one CRTC in parallel with one or more other CRTCs. Likewise, this
means it's also not safe to do so in order to track runtime PM state.
While this code is certainly wrong, so far the only issues I've seen
this cause in the wild is the occasional PM ref unbalance after an
atomic check failure + module reloading (since the PCI device will
outlive nouveau in such scenarios).

So, do this far more elegantly: grab a runtime PM ref across the modeset
and commit tail, then grab/put references for each CRTC enable/disable.
This also ends up being much simpler then the previous broken solution
we had.

Finally, since we've removed all it's users: get rid of
nouveau_drm->have_disp_power_ref.

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