drm/xe/display: add notes about how early a few functions can be called

xe_display_driver_probe_defer() and xe_display_driver_set_hooks() get
called before either struct xe_device or struct intel_display
exist. Make a note of that.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://lore.kernel.org/r/937ea1e16e970a6f6944b94c6a9c216d36e728d1.1747397638.git.jani.nikula@intel.com


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