Move display global state member under drm_i915_private display
sub-struct.

Prefer adding anonymous sub-structs even for single members that aren't
our own structs.

Remove a nearby stale comment while at it.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221109144209.3624739-3-jani.nikula@intel.com