drm/i915/display: Fix macro HAS_ULTRAJOINER

Currently, Ultrajoiner is supported only on Xe2_HPD.
Update the HAS_ULTRAJOINER macro to reflect the same.

v2: Clarify the commit message to specify platform. (Jani)

Bspec: 69556
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Karthik B S <karthik.b.s@intel.com>
Link: https://lore.kernel.org/r/20250611053039.377695-1-ankit.k.nautiyal@intel.com