drm/i915: s/intel_dp_aux_ch/intel_dp_aux_xfer/

Rename intel_dp_aux_ch() to intel_dp_aux_xfer() to better convey
what it actually does.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222181036.15251-6-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #irc