drm/nouveau/kms: move drm_crtc_force_disable_all() earlier

nouveau_display_fini() is responsible for quiescing the hardware, so
this is where such actions belong.

More than that, nouveau_display_fini() switches off the receiving of
sink irqs, which MST will require while shutting down an active head.

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