drm/i915: move and group sagv under display.sagv

Move display sagv 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/d67c632f6bc75f6ce562c28f3521404d30ed48f4.1661346845.git.jani.nikula@intel.com