drm/i915: Add a modeset power domain

We need a power domain for disabling DC5/DC6 around modesets to prevent
confusing the DMC.

Signed-off-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447084107-8521-8-git-send-email-patrik.jakobsson@linux.intel.com