drm/xe/display: use xe->display to decide whether to do anything

Since we only initialize xe->display when xe->info.probe_display, we can
use !xe->display to bail out early. This seems cleaner and more accurate
than relying on xe->info.probe_display, since xe->display may indeed be
NULL.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://lore.kernel.org/r/945d2a987214044a81f4816684972961b772b45a.1747397638.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>