drm/i915: disable PCH ports if needed when disabling a CRTC

Disable any PCH ports associated with a pipe when disabling it.  This
should prevent transcoder disable failures due to ports still being on.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: introduce *_PIPE_ENABLED() macro]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>