drm/i915: use the new dp train delay helpers

Only really required for dp 1.2. I've hoped this would help with some
link training woes I'm fighting, but alas those are only dp 1.1
devices.

Also move a comment that went misplaced in the recent refactorings to
the right spot again.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>