drm/i915: split fdi link training from display vtable.

It may make sense to merge this with display again later,
however the fdi use of the vtable is limited to only a
few generations.

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/7af7359b6cec33bd2d32152893d9a1e8f8cf7f21.1632869550.git.jani.nikula@intel.com