drm/i915: Clean up AUX power domain handling

Introduce intel_display_port_aux_power_domain() which simply returns
the appropriate AUX power domain for a specific port, and then replace
the intel_display_port_power_domain() with calls to the new function
in the DP code. As long as we're not actually enabling the port we don't
need the lane power domains, and those are handled now purely from
modeset_update_crtc_power_domains().

My initial motivation for this was to see if I could keep the DPIO power
wells powered down while doing AUX on CHV, but turns out I can't so this
doesn't change anything for CHV at least. But I think it's still a
worthwile change.

v2: Add case for PORT E. Default to POWER_DOMAIN_AUX_D for now. (Ville)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447682467-6237-1-git-send-email-patrik.jakobsson@linux.intel.com