drm/udl: Drop drm_vblank_cleanup

udl doesn't shut down the display, so stopping the vblank isn't going
to do much good either. Just drop it.

Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-11-daniel.vetter@ffwll.ch