drm/i915/dp: use the new drm helpers for dp i2c-over-aux

The functionality remains largerly the same. The main difference is that
i2c-over-aux defer timeouts are increased to be safe for all use cases
instead of depending on DP device type and properties.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>