drm/i915/dp: use new link training delay helpers

Use the new link training delay helpers, fixing the delays for
128b/132b.

For existing 8b/10b functionality, this will cause additional 1-byte
DPCD reads for LTTPR delays instead of using the cached values. It's
just too complicated to combine generic helpers with local caching in a
sensible way.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/20211014150059.28957-3-jani.nikula@intel.com