drm/i915/tv: After disabling the pipe, use wait_for_vblank_off()

Hopefully this is a contributing factor to the spurious TV detection
repoted by Ivan Bulatovic and others.

References:

  Bug 16871 - "TV1 connected" with no tv
  https://bugzilla.kernel.org/show_bug.cgi?id=16871



Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-by: Ivan Bulatovic <combuster@gmx.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>