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

Move display property 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/14b14f871e322419b10166c1bd8a5a956f5430c8.1661779055.git.jani.nikula@intel.com