drm/i915: Store aux power domain in intel_dp

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: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170222063431.10060-2-ander.conselvan.de.oliveira@intel.com