drm/i915: Don't write the HDMI buffer translation entry when not needed

We don't actually need to write the HDMI entry on DDIs that have no
chance to be used as HDMI ports.

While this patch shouldn't change the current behaviour, it makes
further enabling work easier as we'll have an eDP table filling the full
10 entries.

v2: Rely on the logic from intel_ddi_init() to figure out if the DDI port
    supports HDMI or not (Paulo).

Suggested-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>