drm/i915/opregion: Convert to using native drm_i915_private

Prefer passing struct drm_i915_private to internal interfaces as this
saves us having to dance between drm_device and our native struct. The
savings hare are small (only 70 bytes of unrequired dancing), but
progressive!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464012490-30961-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>