drm/i915: add port assertion check when disabling transcoders

When a transcoder is disabled, any ports pointing at it should also be
disabled.  If they're not, we may fail to disable the transcoder,
leading to blank displays.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>