drm/i915: factor out FDI disable and add FDI assertions

Factor out the FDI disable function (make it a mirror of
ironlake_fdi_enable) and add some FDI related assertions to the FDI
training code (we need an active pipe & plane before we start
transmitting bits).

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