drm/i915: Kill intel_prepare_ddi()

Move the ddi buffer translation programming to occur from the encoder
.pre_enable() hook, for just the ddi port we are enabling. Previously
we used to reprogram the translations for all ddi ports during
init and during power well enabling.

v2: s/intel_prepare_ddi_buffers/intel_prepare_ddi_buffer/ (Daniel)
    Resolve conflicts due to  dev_priv->atomic_cdclk_freq

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>