drm/i915: Move intel_dp->prepare_link_train assignment into ddi code

It's a bit silly to go through intel_dp.c to assign the
prepare_link_train vfunc for ddi platforms when we can just
assign it directly from intel_ddi.c.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190604140214.9947-1-ville.syrjala@linux.intel.com


Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>