drm/xe: move xe_display.[ch] under display/

All the other display related files are under display/ subdirectory,
also move xe_display.[ch] there.

Sort the build list while at it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240122101428.2683468-1-jani.nikula@intel.com