drm/i915: move overlay to display.overlay

Move display overlay 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>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e019fb538536d36bab28feaafe049e0b1726f2d9.1661346845.git.jani.nikula@intel.com