drm/i915: Use the correct power domain for aux ch

Select the aux power domain based on the aux ch rather than based on
the port. Now we can rid ourselves of the port E FIXME as well.

v2: Split from the enum aux_ch patch (Rodrigo)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> #v1
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222181036.15251-1-ville.syrjala@linux.intel.com