drm/i915: Use the correct AUX power domain in TypeC TBT-alt mode

In the TypeC TBT-alt port mode we must use the TBT AUX power domain,
fix that.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190628143635.22066-8-imre.deak@intel.com