drm/i915: Model PSR AUX register selection more like the normal AUX code

v2: Split up the ctl vs. data reg handling like in the normal AUX code

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1447266856-30249-8-git-send-email-ville.syrjala@linux.intel.com