drm/i915: Collect aux ch vfunc setup into intel_dp_aux_init()

Collect all the aux ch vfunc assignments into intel_dp_aux_init()
instead of having it spread around.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222181036.15251-4-ville.syrjala@linux.intel.com