drm/i915: move opregion to display.opregion

Move display opregion related members under drm_i915_private display
sub-struct.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a0ceb5148835fa3e0828786ae491fcd11e2e77ff.1661779055.git.jani.nikula@intel.com