drm/nouveau: Use drm_fb_helper_output_poll_changed()

This driver can use drm_fb_helper_output_poll_changed() instead of
its own nouveau_fbcon_output_poll_changed().

Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171205182504.41923-8-noralf@tronnes.org