drm/i915: Introduce .set_idle_link_train() vfunc

Relocate a bunch of DDI specific code from intel_dp.c to intel_ddi.c
by introducing a .set_idle_link_train() vfunc.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200420200610.31798-3-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>