drm/i915/psr: Do not read PSR2_SU_STATUS on AlderLake and onwards

Bspec comment on PSR2_SU_STATUS:

"This register has been tied-off since DG2/ADL-P (it returns zeros only)
and it has been removed on Xe2_LPD."

v2: fix inversed logic

Bspec: 69889
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250516063019.2126702-1-jouni.hogander@intel.com