drm/i915: disable DDI_BUF_CTL at the correct time

And also properly wait for its idle bit.

You may notice that DDI_BUF_CTL is enabled in .enable but disabled in
.post_disable instead of .disable. Yes, the mode set sequence is not
exactly symmetrical, but let's assume the spec is correct unless we
can prove it's wrong.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>