drm/i915: Explicitly use ddi buf trans entry 9 for hdmi

When the DDI port is in HDMI/DVI mode, it automagically uses the buffer
translations values from entry 9. Let's make that explicit in the code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468328376-6380-6-git-send-email-ville.syrjala@linux.intel.com