drm/i915: Consult aux_ch instead of port in ->get_aux_clock_divider()

While it seems totally unlikely that any system would mix a cpu/north
aux channel with a pch/south port (or vice versa) we should still
consult intel_dp->aux_ch rather than encoder->port when figuring out
which clock is actually used by the aux ch.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222181036.15251-5-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #irc