drm/i915/psr: New power domain for AUX IO.

PSR on CNL requires AUX IO wells to be kept on and the existing AUX domain
for AUX-A enables DC_OFF well too. This is not required, so add a new
AUX_IO_A domain for AUX-A to allow DC states to remain enabled. Other AUX
channels re-use the existing AUX domains.

v4: Reword comment (Rodrigo and Ville)
    Rename _get and _put functions to include aux_io substring(Rodrigo)
    Remove unnecessary diff that got included.
v3: Extract aux domain selection into a function (Ville)
v2: Add AUX IO domain only for AUX-A
    Rebased on top of Ville's AUX series.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Suggested-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180223221520.18464-1-dhinakaran.pandiyan@intel.com