drm/nouveau/kms/nv50-: Report possible_crtcs incorrectly on mstos, for now

This commit is seperate from the previous one to make it easier to
revert in the future. Basically, while working on making MSTOs per-head
as opposed to per-head-per-connector I discovered these lovely issues:

https://gitlab.freedesktop.org/xorg/xserver/merge_requests/277
https://gitlab.gnome.org/GNOME/mutter/issues/759



Note as well that Intel already has a temporary workaround for this in
their kernel driver. So, unfortunately we need to follow suit to avoid
causing a regression in userspace. Once these issues get fixed, this
commit should be reverted.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>