drm/i915/dp: use the new drm helpers for dp aux

Functionality remains largely the same as before.

Note that the retry loops and native reply handling all moved into the
core drm helper functions now.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Fix up the stray ; Rodrigo spotted in his review and add a
note to the commit message to answer Rodrigo's question in his review.]
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>