drm/i915/dp: convert intel_dp_link_training.[ch] to struct intel_display

Going forward, struct intel_display shall replace struct
drm_i915_private as the main display device data pointer type. Convert
intel_dp_link_training.[ch] to struct intel_display.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/72b202e75f5a7ecc84a906f1c49d21dbe24fb7c2.1725012870.git.jani.nikula@intel.com