drm/nouveau: Drop drm_vblank_cleanup

nouveau_display_vblank_fini is called in the load error path (where it
doesn't matter) and module unload (where vblanks have been shut down
correctly already through drm_vblank_off), we can drop it.

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-8-daniel.vetter@ffwll.ch