drm/i915: move quirks under display sub-struct

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

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

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/c4a1a5657023efe24a362c67daf79260f179f0eb.1661779055.git.jani.nikula@intel.com