drm/i915: Add enum aux_ch and clean up the aux init to use it

Since we no longer have a 1:1 correspondence between ports and AUX
channels, let's give AUX channels their own enum. Makes it easier
to tell the apples from the oranges, and we get rid of the
port E AUX power domain FIXME since we now derive the power domain
from the actual AUX CH.

v2: Rebase due to AUX F
v3: Split out the power domain fix (Rodrigo)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> #v2
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v2
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222181036.15251-2-ville.syrjala@linux.intel.com