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