drm/i915: Store encoder power domain in struct intel_encoder

The encoder power domain is obviously tied to the encoder, so store it
in struct intel_encoder. This 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/20170222063431.10060-3-ander.conselvan.de.oliveira@intel.com