drm/xe/display: move xe->display initialization to xe_display_probe()

drm/xe/display: move xe->display initialization to xe_display_probe()

The future goal is to have intel_display_device_probe() create struct
intel_display. As the first step, postpone xe->display initialization
right before that call. This is the same location as in i915.

There's a subtle functional change here: xe->display will now be
initialized only if xe->info.probe_display.

The xe_display_create() function becomes empty, and can be removed. Move
its documentation to xe_display_probe()

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://lore.kernel.org/r/6c3075739d84cecea258d686c3ef38455a61191c.1747397638.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>