drm/i915/opregion: move acpi notifier to dev_priv

Get rid of the silly static variable.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180613113927.30033-1-jani.nikula@intel.com