drm/i915: split DP link training across panel power sequencing

Mode set sequence requires that we start training, then enable the
panel, then complete training.  So split the DP training function into
two parts; the first enables the DP port and sets training pattern 1 and
the second completes the training.

As part of this, remove some redundant function args from the various DP
handling functions and use the intel_dp fields everywhere we can.
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: removed first ironlake_edp_backlight_on() on advice of jbarnes]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>