The aux power domain only makes sense in the DP code. Storing it in
struct intel_dp avoids some indirection.

v2: Rebase
Signed-off-by: NAnder Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: NImre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170222063431.10060-2-ander.conselvan.de.oliveira@intel.com