drm/i915/display: add intel_fdi_link_train wrapper.

This wraps the fdi link training vfunc to make it clearer.

Suggested by Jani.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1bb978bcb6f16fbdaf08f2800a179b774525b59e.1632869550.git.jani.nikula@intel.com