drm/i915/display: Use intel_attached_dp instead of local implementation

drm/i915/display: Use intel_attached_dp instead of local implementation

Link training code contains local "connector to intel_dp"
implementation. This was added due to missing MST handling in
intel_attached_dp. Missing MST support is now fixed in
intel_attached_dp. We can now change link training code to use that instead
of local intel_connector_to_intel_dp.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240906070033.289015-3-jouni.hogander@intel.com