drm/i915/dp: move dp aux ch register init to aux init

Do a slight rearrangement of the switch to prep for follow-up.

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>